Acidic superabsorbent hydrogels

ABSTRACT

The present invention relates to novel hydrophilic swellable addition polymers comprising improved odor control, a process for their preparation and their use for absorbing aqueous fluids and a process for screening superabsorbents.

[0001] The present invention relates to novel hydrophilic swellableaddition polymers comprising improved odor control, their preparationand their use for absorbing aqueous fluids and also a process forscreening superabsorbents.

[0002] More particularly, the present invention relates to acidicsuperabsorbent hydrogels having a pH≦5.7, i.e., hydrogels based onpolyacrylic acid whose degree of neutralization is preferably ≦60 mol %,processes for their preparation and their use in hygiene articles.

[0003] Swellable hydrogel forming addition polymers, known assuperabsorbent polymers or SAPs, are known from the prior art. They arenetworks of flexible hydrophilic addition polymers, which can be bothionic and nonionic in nature. They are capable of absorbing and bindingaqueous fluids by forming a hydrogel and therefore are preferentiallyused for manufacturing tampons, diapers, sanitary napkins, incontinencearticles, training pants for children, insoles and other hygienearticles for the absorption of body fluids. Superabsorbents are alsoused in other fields of technology where fluids, especially water oraqueous solutions, are absorbed. These fields include for examplestorage, packaging, transportation (packaging material forwater-sensitive articles, for example flower transportation, shockprotection); food sector (transportation of fish, fresh meat; absorptionof water, blood in fresh fish/meat packs); medicine (wound plasters,water-absorbent material for burn dressings or for other weepingwounds), cosmetics (carrier material for pharmaceuticals andmedicaments, rheumatic plasters, ultrasound gel, cooling gel, cosmeticthickeners, sunscreen); thickeners for oil/water or water/oil emulsions;textiles (gloves, sportswear, moisture regulation in textiles, shoeinserts); chemical process industry applications (catalyst for organicreactions, immobilization of large functional molecules (enzymes),adhesive for agglomerations, heat storage media, filtration aids,hydrophilic component in polymer laminates, dispersants, liquefiers);building and construction, installation (powder injection molding,clay-based renders, vibration-inhibiting medium, assistants in relationto tunneling in water-rich ground, cable sheathing); water treatment,waste treatment, water removal (de-icers, reusable sandbags); cleaning;agriculture industry (irrigation, retention of meltwater and dewprecipitates, composting additive, protection of forests against fungaland insect infestation, delayed release of active ingredients toplants); fire protection (flying sparks)(covering houses or house wallswith SAP gel, since water has a very high heat capacity, ignition can beprevented; spraying of SAP gel in the case of fires such as for exampleforest fires); coextrusion agent in thermoplastic polymers(hydrophilicization of multilayer films); production of films andthermoplastic moldings capable of absorbing water (for exampleagricultural films capable of storing rain and dew water; SAP-containingfilms for keeping fresh fruit and vegetables which can be packed inmoist films; the SAP stores water released by the fruit and vegetableswithout forming condensation droplets and partly reemits the water tothe fruit and vegetables, so that neither fouling nor wilting occurs;SAP-polystyrene coextrudates for example for food packs such as meat,fish, poultry, fruit and vegetables); carrier substance inactive-ingredient formulations (drugs, crop protection). Within hygienearticles, superabsorbents are generally positioned in an absorbent corewhich, as well as SAP, comprises other materials, including fibers(cellulose fibers), which act as a kind of liquid buffer tointermediately store the spontaneously applied liquid insults and areintended to ensure efficient channelization of the body fluids in theabsorbent core toward the superabsorbent.

[0004] The current trend in diaper design is toward ever thinnerconstructions having a reduced cellulose fiber content and an increasedhydrogel content. The trend toward ever thinner diaper constructions hassubstantially changed the performance profile required of the waterswellable hydrophilic polymers over the years. Whereas at the start ofthe development of highly absorbent-hydrogels it was initially solelythe very high swellability on which interest focused, it wassubsequently determined that the ability of the superabsorbent totransmit and distribute fluid is also of decisive importance. It hasbeen determined that conventional superabsorbents greatly swell at thesurface on wetting with liquid, so that transportation of liquid intothe particle interior is substantially compromised or completelyprevented. This trait of superabsorbents is known as gel blocking. Thegreater amount of polymer per unit area in the hygiene article must notcause the swollen polymer to form a barrier layer to subsequent fluid. Aproduct having good transportation properties will ensure optimalutilization of the entire hygiene article. This prevents the phenomenonof gel blocking, which in the extreme case will cause the hygienearticle to leak. Fluid transmission and distribution is thus of decisiveimportance with regard to the initial absorption of body fluids.

[0005] Good transportation properties are possessed for example byhydrogels having high gel strength in the swollen state. Gels lacking instrength are deformable under an applied pressure, for example pressuredue to the bodyweight of the wearer of the hygiene article, and clog thepores in the SAP/cellulose fiber absorbent and so prevent continuedabsorption of fluid. Enhanced gel strength is generally obtained througha higher degree of crosslinking, although this reduces retentionperformance. An elegant way to enhance gel strength is surfacepostcrosslinking. In this process, dried superabsorbents having anaverage crosslink density are subjected to an additional crosslinkingstep. The process is known to one skilled in the art and described inEP-A-0 349 240. Surface postcrosslinking increases the crosslink densityin the sheath of the superabsorbent particle, whereby the absorbencyunder load is raised to a higher level. Whereas the absorption capacitydecreases in the superabsorbent particle sheath, the core has animproved-absorption capacity (compared to the sheath) owing to thepresence of mobile polymer chains, so that sheath construction ensuresimproved fluid transmission without occurrence of the gel blockingeffect. It is perfectly desirable for the total capacity of thesuperabsorbent to be occupied not spontaneously but with time delay.Since the hygiene article is generally repeatedly insulted with urine,the absorption capacity of the superabsorbent should sensibly not beexhausted after the first disposition.

[0006] When hydrogels are used in the hygiene sector, they becomeexposed to body fluids such as urine or menses. Body fluids generallycontain malodorous components of the amine or fatty acid type, whichappear alongside the organic components anyhow present, for example,amines, acids, aldehydes, ketones, phenols, polycyclics, indoles,aromatics, polyaromatics, etc., that are responsible for unpleasant bodyodors. Odor development takes place in two stages, first in the courseof exudation from the body region and then when the fluid has alreadybeen present in the absorption medium for a defined time. Both odorfactors have to be eliminated, since it is undesirable for cost reasonsto change the hygiene article after every absorption process.

[0007] The literature on odor control in the hygiene sector reveals thefollowing approaches:

[0008] Odor control coupled with simultaneous absorption by addition ofinert inorganic substances having a large surface area, generally as asolid onto the surface of powders or granules for manufacturingabsorbent polymers. Zeolites, active carbon, bentonites, finely dividedamorphous silicas such as AEROSIL® or CAB-O-SIL® are used here.

[0009] Addition of substances capable of complexing with organicmolecules or with metal ions present in the body fluid to prevent thedevelopment of unpleasant odors. This preferably takes the form of theuse of cyclodextrins (any modification of unsubstituted cyclodextrinswhich contains from 6 to 12 glucose units, for examplealpha-cyclodextrin and beta-cyclodextrin, gamma-cyclodextrin and/orderivatives and/or mixtures thereof. Mixtures of cyclodextrins arepreferred, since they provide broader complexation of organic moleculesover a wider molecular weight range. Cyclodextrins are used from 0.1% toabout 25%, preferably from 1% to about 20%, more preferably from 2% toabout 15% and especially from 3 to 10%, based on the total weight of thecomposition.

[0010] Cyclodextrins are added in small particle size (usually less than12 μm) to offer a large surface area for odor elimination. Furthercomplexing agents are aminopolycarboxylic acids and their salts,ethylenediaminetetraacetate EDTA ethylenediaminepentamethylenephosphonicacid, ethylenediaminetetramethylenephosphonic acid, aminophosphates,polyfunctional aromatics, N,N-disuccinic acid.

[0011] Masking of unpleasant odors by addition of perfumes ordeodorants. These are added in free form or in encapsulated form (forexample in cyclodextrins). The latter form makes it possible to releasethe perfume with a time delay.

[0012] Nonlimiting examples of perfumes are allyl caproate,allylcyclohexane acetate, allylcyclohexane propionate, allyl heptanoate,amyl acetate, amyl propionate, anetole, anisole, benzaldehyde, benzylacetate, benzylacetone, benzyl alcohol, benzyl butyrate, benzyl formate,benzyl isovalerate, benzyl propionate, butyl benzoate, butyl caproate,camphor, cis-3-hexenyl acetate, cis-3-hexenyl butyrate, cis-3-hexenylcaproate, cis-3-hexenyl valerate, citronellol, citronellyl derivates,Cyclal C, cyclohexylethyl acetate, 2-decenal, decylaldehyde,dihydromyrcenol, dimethylbenzylcarbinol and derivatives thereof,dimethyloctanol, diphenyl oxide, ethyl acetate, ethyl acetoacetate,ethyl amyl ketone, ethyl benzoate, ethyl butyrate, ethyl hexyl ketone,ethyl phenylacetate, eucalyptol, fenchyl acetate, fenchyl alcohol,tricyclodecenyl acetate, tricyclodecenyl propionate, geraniol, geranylderivatives, heptyl acetate, heptyl isobutyrate, heptyl propionate,hexenol, hexenyl acetate, hexenyl isobutyrate, hexyl acetate, hexylformate, hexyl isobutyrate, hexyl isovalerate, hexyl neopentanoate,hydroxycitronellal, α-ionone, β-ionone, γ-ionone, isoamyl alcohol,isobornyl acetate, isobornyl propionate, isobutyl benzoate, isobutylcaproate, isononyl acetate, isononyl alcohol, isomenthol, isomenthone,isononyl acetate, isopulegol, isopulegyl acetate, isoquinoline,dodecanal, lavandulyl acetat, ligustral, δ-limonene, linalool andderivatives, menthone, menthyl acetate, methylacetophenone, methyl amylketone, methyl anthranilate, methyl benzoate, methyl- benzylacetate,methylchavicol, methyleugenol, methylheptenone, methyl heptynecarbonate,methyl heptyl ketone, methyl hexyl ketone, methylnonylacetaldehyde,α-iso“γ”methylionone, methyloctylacetaldehyde., methyl octyl ketone,methylphenylcarbinyl acetate, methyl salicylate, myrcene, myrcenylacetate, neral, nerol, neryl acetate, nonalactone, nonyl butyrate, nonylalcohol, nonyl acetate, nonylaldehyde, octalactone, octyl acetate, octylalcohol, octylaldehyde, d-limonene, p-cresol, p-cresyl methyl ether,p-cymene, p-isopropyl-p-methylacetophenone, phenethyl anthranilate,phenoxyethanol, phenylacetaldehyde, phenylethyl acetate, phenylethylalcohol, phenylethyldimethylcarbinol, α-pinene, β-pinene, α-terpinene,γ-terpinene, terpineol, terpinyl acetate, terpinyl propionate,tetrahydrolinalool, tetrahydromyrcenol, thymol, prenyl acetate, propylbutyrate, pulegone, safrole, δ-undecalactone, γ-undecalactone,undecanal, undecyl alcohol, veratrol, verdox, vertenex, viridine.

[0013] Addition of urease inhibitors to control the formation oractivity of enzymes responsible for the cleavage of urea into ammoniaand hence for odor development.

[0014] Addition of antimicrobial substances. Enzymes control bacterialgrowth and thereby minimize odor development due to bacterialdegradation processes (e.g., oxidoreductase+mediator). Examples ofantimicrobial substances include quaternary ammonium compounds, phenols,amides, acids and nitro compounds and also mixtures thereof.

[0015] Examples of quaternary ammonium compounds include2-(3-anilinovinylul)3,4-dimethyloxazolinium iodide, alkylisoquinoliumbromide, benzalkonium chloride, benzethonium chloride, cetylpyridiniumchloride, chlorhexidine gluconate., chlorhexidine hydrochloride,lauryltrimethylammonium compounds, methylbenzethonium chloride,stearyltrimethylammonium chloride, 2,4,5-trichlorophenoxide and alsomixtures thereof.

[0016] Examples of phenols include benzyl alcohol, p-chlorophenol,chlorocresol, chloroxylenol, cresol, o-cymen-5-ol (BIOSOL),hexachlorophene, chinokitiol, isopropylmethylphenol, parabens (withmethyl, ethyl, propyl, butyl, isobutyl, isopropyl, and/or sodium methylsubstituents), phenethyl alcohol, phenol, phenoxyethanol,o-phenylphenol, resorcinol, resorcinol monoacetate, sodium parabens,sodium phenolsulfonate, thioxolone, 2,4,4′-trichloro-2′-hydroxydiphenylether, zinc phenolsulfonate, di-tert-butylphenol, hydroquinone, BHT andalso mixtures thereof.

[0017] Examples of amides include diazolidinylurea,2,4-imidazolidinedione (HYDATOIN), 3,4,4′-trichlorocarbanilide,3-trifluoromethyl-4,4′-dichlorocarbanilide, undecylenoic acidmonoethanolamide and also mixtures thereof.

[0018] Examples of acids-include benzoates, benzoic acid, citric acid,dehydroacetic acid, potassium sorbate, sodium citrates, sodiumdehydroacetate, sodium salicylate, sodium salicylic acid, sorbitanicacid, undecylenoic acid, zinc undecylenate, zinc oxide, zincphenolsulfonate, ascorbic acid, acetylsalicylic acid, salicylaldehyde,salicylic acid derivatives, adipic acid, adipic acid derivatives andalso mixtures thereof.

[0019] Examples of nitro compounds include2-bromo-2-nitro-2,3-propanediol (BRONOPOL), methyldibromoglutaronitrileand propylene glycol (MERGUARD) and also mixtures thereof.

[0020] In addition the following compounds are useful as biocides:2,5-dimethoxytetrahydrofuran, 2,5-diethoxytetrahydrofuran,2,5-dimethoxy-2,5-dihydrofuran, 2,5-diethoxy-2,5-dihydrofuran,succinaldehyde, glutaraldehyde, glyoxal, glyoxylic acid,hexahydrotriazine, tetrahydro-3,5-dimethyl-2H-1,3,5-thiadiazine-2-thione(Dazomet), 2,4-dichlorobenzyl alcohol, benzalkonium chloride,chlorhexidine gluconate, triclosan.

[0021] Use of microcapsules which release the active substance oncontact with moisture.

[0022] Use of transition metal compounds (Cu, Ag, Zn).

[0023] As well as the classes of compounds mentioned, useful odorcontrol compounds further include the following: peroxides, bicarbonate,triclosan, plant extracts, ethereal oils, boron compounds,poly-alpha-amino acids (polylysine), imides, polyimides, PVP-iodine, useof certain polymeric substances such as chitosan, polyglycosides,oxidizing agents, cyclophanes.

[0024] In general, however, the addition of odor inhibitors will have anadverse effect on the absorption profile of superabsorbent hydrogels.The separate installation of the odor-inhibiting or deodorizingcomponent system and of the superabsorbent material in the hygienearticle generally reduces the absorption capacity. Combinations,generally exhibit a worse performance profile than the individualcomponents as such. Moreover, the individual components may separateunder mechanical stress of the kind exerted in the course of the wearingof the hygiene article for example. If, however, blends are preparedwhere the additives adhere to the surface of the dried superabsorbentpolymers, the surface properties of the absorbent hydrogels may bechanged without the intrinsic absorption properties being impaired. Theresult may be for example a hydrophilicization or a hydrophobicization,which primarily affects the fluid uptake rate. All these polymers,however, generally have in common that the permeability through swollengel is unsatisfactory.

[0025] Odor control on using acidic hydrogels in hygiene articles isgood. However, they exhibit a worse absorption profile than is the casewith pH neutral products.

[0026] The superabsorbent hydrogels used in the hygiene sector atpresent are addition polymers possessing a degree of neutralization inthe range from 60 to 80 mol %, based on the polymerized acid-functionalmonomer units. However, it was found with regard to odor control that ahigher pH will generally favor bacterial growth. In the process, theurea in the urine is increasingly split by urease into carbon dioxideand urea, which leads a further increase in the pH. This in turnreinforces bacterial growth, and enzyme activity is further increased.One consequence of the raised pH is the occurrence of soft skin, makingthe skin more susceptible to bacterial colonization. This resultsdirectly in skin irritation which will preclude the wearing of thehygiene article for a prolonged period.

[0027] The manufacturing process of completely acidic hydrogel formingonomers is known and has been repeatedly described in the literature. EP205 674 A1 discloses the preparation of completely acidic additionpolymers at temperatures from 0 to 100° C., and preferably from 5 to 40°C., which are adjusted by subsequent partial neutralization of thehydrogels. The addition polymers are notable for improved absorptioncapacity and also for lower extractables. Similarly, U.S. Pat. No.5,145,906 and EP 530 438 B1 disclose the preparation of addition polymergels from acrylic acid with polymers containing water-soluble hydroxylgroups in an acidic polymerization, i.e., without neutralization of themonomers, which gels are subsequently comminuted and; partially orcompletely neutralized by means of aqueous bases and subsequentlysubjected to postcrossslinking. However, the processes all-have incommon that the polymerization of the monomer solution as shown in EP467 073 A1 proceeds very slowly, so that only a batch process ispossible. Increasing the amount of initiator or raising thepolymerization temperature has an adverse effect on the absorptionprofile of the hydrogels. Moreover, there are appreciable problemsduring the manufacturing process with the subdivision of the completelyacidic polymer gel, and the neutralization which is carried outsubsequently merely takes place under diffusion control, so that thepolymer surface has a base excess. Hydrogels prepared by acidicpolymerization generally exhibit worse absorbencies under load and alsoan appreciable rewet, and this has an adverse effect on the use in thehygiene sector.

[0028] On the other hand, there are processes in existence where themonomer solution has already been subjected to a partial neutralizationand whose addition polymer gels are lastly adjusted to the desireddegree of neutralization following the polymerization. For instance, DE195 29 348 reports a process wherein the monomer solution is 5-30 mol %,preferably 5-20 mol % and particularly preferably 5-10 mol %neutralized, based on the acid-functional monomers, whereupon thepartially neutralized monomer batch is polymerized and subsequently theaddition polymer is further neutralized until at least 50 mol % of theacid groups present therein are neutralized. This process providesaddition polymers having a high retention value and a high sorbencyunder constant and increasing pressure and also having a low level ofextractables. EP 0 583 178 B1, in contrast, proposes a process forpreparing superabsorbent powders consisting of partially neutralizedpolyacrylic acids by a sequential inverse suspension polymerization oftwo charges having different degrees of neutralization (Charge I: degreeof neutralization 90-100%, Charge III: degree of neutralization 50-60%),charge II being absorbed before polymerization by the polymer of chargeI.

[0029] None of the cited processes generates hydrogel forming addition 5polymers which confer all the absorption profile advantages of theoptimized skin pH neutral superabsorbent on acidic addition polymers, sothat a distinct odor control unit is required in each case.

[0030] GB 2326348 A and WO 00/35502 impressively report the relationshipbetween bacterial growth and the pH of the surrounding medium. Effectivecontrol of bacterial growth is said to be possible only at pH valuesbetween 3.5 and 4.9, preferably between 4.1 and 4.7. This wouldcorrespond to a degree of neutralization in the range from 20 to 35 mol%. However, these references disregard the quantification of theabsorption profile underlying hydrogels of this degree ofneutralization. GB 2326348 A mentions in general that the lowering ofthe absorption capacity may be compensated by increasing the amount ofhydrogel. WO 00/35502, in contrast, proposes installing the hydrogelforming substance in a layer within the hygiene article that is closerto the skin.

[0031] It is an object of the present invention to provide modifiedsuperabsorbents (and a process for their preparation) which when used inhygiene articles comprise improved odor inhibition coupled with theexcellent absorption profile on the part of the superabsorbent hydrogelmaterial. This hydrogel material shall preferably possess rapid swellingand good transportation properties coupled with a high ultimateabsorption capacity and also improved gel strength and higherelectrolyte tolerance without exhibiting the adverse effect of gelblocking. The high absorption performance sought shall not besubstantially impaired by the odor control system, as has heretoforebeen the case in the prior art with odor control-in the hygiene sector.

[0032] We have found that this object is achieved, surprisingly,preferably by the sole use of superabsorbent hydrogels whose acid groupshave been neutralized to a lesser degree by the addition of bases beforeor after the polymerization. The range for the 40 degree ofneutralization extends from 5 to 60 mol %, preferably from 10 to 40 mol% and particularly preferably from 20 to 30 mol %, based on theacid-functional monomers.

[0033] The present invention accordingly provides for the preparationand use of acidic superabsorbent hydrogels of the above degree ofneutralization where preferably no further substances have been addedfor odor inhibition. Surprisingly, furthermore, we have developed aparameter to quantify the absorption profile—namely the pH absorbencyindex PH_(AI)—which permits an informative assessment of newly developedhydrogel material. Furthermore, the PH_(AI) parameter makes it possibleto provide a simple test method for screening and optimizing newmaterials by summarizing and weighting the relevant characteristics ofthe superabsorbents.

[0034] The preparation and optimization of novel materials increasinglyutilizes parallel syntheses with or without the assistance of robotsynthesizers. The material obtained has to be performance tested inorder that the next generation of materials may be further optimizedwith regard to one or more parameters. The present invention provides inthe pH absorbency index a test parameter which permits simple yetinformative assessment of superabsorbents. Moreover, the determinationof the individual parameters needed to calculate the index can beminiaturized and parallelized, so that high throughput screening (HTS)of the kind familiar in drug and pesticide discovery research can becarried out. The pH, CFC and AUL-0.7 psi indices can either bedetermined as described in the description part or estimated byconducting equivalent determinations. Such equivalents include forexample the calculation of the degree of neutralization of the monomerused, subsequent neutralization of the gel with known amount ofneutralizing agent etc. for the pH; the AUL-0.7 psi can likewise bereplaced by similar tests at other pressures.

[0035] The invention accordingly provides a process for screeningsuperabsorbents which comprises determining or estimating the pH, CRCand AUL 0.7 psi of a plurality of absorbent samples and determining orestimating the pH absorbency index therefrom. Repeated (iterative)application of this screening process by varying one or more opposing ornonopposing parameters makes it possible to optimize the superabsorbentthrough varied production processes from each measurement of the pHabsorbency index. The novel superabsorbents optimized by this processare likewise claimed. A method for determining the swellability ofpolymer gels under pressure is described in PCT/EP/01/12959 and can beappropriately adapted. The disclosure content of this PCT specificationis incorporated in the disclosure content of the present invention.

[0036] Commercially available superabsorbent material has a pH of 6. Itis known that the absorption capacity of hydrogels is at a maximum atthis pH. Moreover, a high swell rate is observed at this pH. However,the pH at this good absorption performance is distinctly above the pH ofthe skin, so that the skin may be sensitized and irritated.

[0037] The absorption performance at pH 6 can be quantified in terms. ofthe absorbency under load (0.7 psi) (AUL) of 25 g/g for example and acentrifuge retention capacity CRC of 35 g/g for example. Since the abovenumerical values correlate via the pH, they are simple to combine intoone parameter. The starting point for the calculation is the differencein the pH from pH 7:

Δ pH=7−pH of product

[0038] The parameter is then calculated therefrom as pH absorbencyindex=PH_(AI):

PH _(AI) =Δ pH·(AUL _(0.7psi) +CRC)

[0039] It is a good way of quantifying the performance capability of thesuperabsorbent hydrogel material by stating a single numerical value.

[0040] Commercially available polymer material with pH=6,AUL_(0.7psi)=25 g/g and CRC=35 g/g has a pH absorbency index (calculatedas above) of

PH _(AI)=(7−6)·(25+35)=60

[0041] Since commercial products for the hygiene sector constituteoptimized superabsorbent hydrogel material, a summation factor of 60 maybe considered to be the lower limit for the performance capability of asuperabsorbent product.

[0042] It has been determined that conventional commercial hydrogelmaterial without any odor control has a summation factor in the rangefrom 55 to 80, predominantly from 60 to 70.

[0043] The present invention accordingly provides acidic hydrogelforming polymers which are capable of absorbing aqueous fluids and whoseacid functional monomers have been partially neutralized in a smallamount by the addition of bases before or after polymerization. Thepartial neutralization range extends from 5 to 60 mol %, preferably from10 to 40 mol % and particularly preferably from 20 to 30 mol %, based onthe acid functional monomers. More particularly, the present inventionprovides hydrogel forming polymers capable of absorbing fluids, having apH of ≦5.7, and having a pH absorbency index of at least 80.

[0044] Acidic hydrogel material according to the invention, in contrast,comprises complete odor control due to effective control of bacterialgrowth and high absorption performance coupled with a summation factorof above 100, for example in the range from 100 to 130.

[0045] Experimental Part

[0046] Methods of Making

[0047] a) Monomers Used

[0048] Hydrogel-forming polymers are in particular polymers of(co)polymerized hydrophilic monomers, graft (co)polymers of one or morehydrophilic monomers on a suitable grafting base, crosslinked celluloseor starch ethers, crosslinked carboxymethylcellulose, partiallycrosslinked polyalkylene oxide or natural products that swell in aqueousfluids, for example guar derivatives, alginates and carrageenans.

[0049] Suitable grafting bases can be of natural or synthetic origin.Examples are starch, cellulose or cellulose derivatives and also otherpolysaccharides and oligosaccharides, polyvinyl alcohol, polyalkyleneoxides, especially polyethylene oxides and polypropylene oxides,polyamines, polyamides and also hydrophilic polyesters. Suitablepolyalkylene oxides have for example the formula

[0050] where

[0051] R¹ and R² are independently hydrogen, alkyl, alkenyl or acryl,

[0052] X is hydrogen or methyl and

[0053] n is an integer from 1 to 10 000.

[0054] R¹ and R² are each preferably hydrogen, (C₁-C₄)-alkyl,(C₂-C₆)-alkenyl or phenyl.

[0055] Preferred hydrogel-forming polymers are crosslinked polymershaving acid groups which are predominantly in the form of their salts,generally alkali metal or ammonium salts. Such polymers swellparticularly strongly on contact with aqueous fluids to form gels.

[0056] Preference is given to polymers which are obtained bycrosslinking polymerization or copolymerization of acid-functionalmonoethylenically unsaturated monomers or salts thereof. It is furtherpossible to copolymerize these monomers without crosslinker and tocrosslink them subsequently.

[0057] Examples of such monomers bearing acid groups aremonoethylenically unsaturated C₃- to C₂₅-carboxylic acids or anhydridessuch as acrylic acid, methacrylic acid, ethacrylic acid, α-chloroacrylicacid, crotonic acid, maleic acid, maleic anhydride, itaconic acid,citraconic acid, mesaconic acid, glutaconic acid, aconitic acid andfumaric acid. It is also possible to use monoethylenically unsaturatedsulfonic or phosphonic acids, for example vinylsulfonic acid,allylsulfonic acid, sulfoethyl acrylate, sulfo methacrylate, sulfopropylacrylate, sulfopropyl methacrylate,2-hydroxy-3-acryloyloxypropylsulfonic acid,2-hydroxy-3-methacryloyloxypropylsulfonic acid, vinylphosphonic acid,allylphosphonic acid, styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid. The monomers may be usedalone or mixed.

[0058] Preferred monomers used are acrylic acid, methacrylic acid,vinylsulfonic acid, acrylamidopropanesulfonic acid or mixtures thereof,for example mixtures of acrylic acid and methacrylic acid, mixtures ofacrylic acid and acrylamidopropanesulfonic acid or mixtures of acrylicacid and vinylsulfonic acid.

[0059] To optimize properties, it can be sensible to use additionalmonoethylenically unsaturated compounds which do not bear an acid groupbut are copolymerizable with the monomers bearing acid groups. Suchcompounds include for example the amides and nitriles ofmonoethylenically unsaturated carboxylic acid, for example acrylamide,methacrylamide and N-vinylformamide, N-vinylacetamide,N-methyl-N-vinylacetamide, acrylonitrile and methacrylonitrile. Examplesof further suitable compounds are vinyl esters of saturated C₁- toC₄-carboxylic acids such as vinyl formate, vinyl acetate or vinylpropionate, alkyl vinyl ethers having at least 2 carbon atoms in thealkyl group, for example ethyl vinyl ether or butyl vinyl ether, estersof monoethylenically unsaturated C₃- to C₆-carboxylic acids, for exampleesters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylicacid or maleic acid, monoesters of maleic acid, for example methylhydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone orN-vinylcaprolactam, acrylic and methacrylic esters of alkoxylatedmonohydric saturated alcohols, for example of alcohols having from 10 to25 carbon atoms which have been reacted with from 2 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol, and alsomonoacrylic esters and monomethacrylic esters of polyethylene glycol orpolypropylene glycol, the molar masses (Mn) of the polyalkylene glycolsbeing up to 2 000, for example. Further suitable monomers are styreneand alkyl-substituted styrenes such as ethylstyrene ortert-butylstyrene.

[0060] These monomers without acid groups may also be used in mixturewith other monomers, for example mixtures of vinyl acetate and2-hydroxyethyl acrylate in any proportion. These monomers without acidgroups are added to the reaction mixture in amounts within the rangefrom 0 to 50% by weight, preferably less than 20% by weight.

[0061] Preference is given to crosslinked polymers of monoethylenicallyunsaturated monomers which bear acid groups and which are optionallyconverted into their alkali metal or ammonium salts before or afterpolymerization and of 0-40% by weight, based on their total weight, ofmonoethylenically unsaturated. monomers which do not bear acid groups.

[0062] Preference is given to crosslinked polymers of monoethylenicallyunsaturated C₃- to C₁₂-carboxylic acids and/or their alkali metal orammonium salts. Preference is given in particular to crosslinkedpolyacrylic acids where 5-30 mol %, preferably 5-20 mol % andparticularly preferably 5-10 mol % of their acid groups, based on themonomers containing acid groups, are present as alkali metal or ammoniumsalts.

[0063] Possible crosslinkers include compounds containing at least twoethylenically unsaturated double bonds. Examples of compounds of thistype are N,N′-methylenebisacrylamide, polyethylene glycol diacrylatesand polyethylene glycol dimethacrylates each derived from polyethyleneglycols having a molecular weight of from 106 to 8 500, preferably from400 to 2 000, trimethylolpropane triacrylate, trimethylolpropanetrimethacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate, propylene glycol diacrylate, propylene glycoldimethacrylate, butanediol diacrylate, butanediol dimethacrylate,hexanediol diacrylate, hexanediol dimethacrylate, allyl methacrylate,diacrylates and dimethacrylates of block copolymers of ethylene oxideand propylene oxide, polyhydric alcohols, such as glycerol orpentaerythritol, doubly or more highly esterified with acrylic acid ormethacrylic acid, triallylamine, dialkyldiallylammonium halides such asdimethyldiallylammonium chloride and diethyldiallylammonium chloride,tetraallylethylenediamine, divinylbenzene, diallyl phthalate,polyethylene glycol divinyl ethers of polyethylene glycols having amolecular weight of from 106 to 4 000, trimethylolpropane diallyl ether,butanediol divinyl ether, pentaerythritol triallyl ether, reactionproducts of 1 mol of ethylene glycol diglycidyl ether or polyethyleneglycol diglycidyl ether with 2 mol of pentaerythritol triallyl ether orallyl alcohol, and/or divinylethyleneurea. Preference is given to usingwater-soluble crosslinkers, for example N,N′-methylenebisacrylamide,polyethylene glycol diacrylates and polyethylene glycol. dimethacrylatesderived from addition products of from 2 to 400 mol of ethylene oxidewith 1 mol of a diol or polyol, vinyl ethers of addition products offrom 2 to 400 mol of ethylene oxide with 1 mol of a diol or polyol,ethylene glycol diacrylate, ethylene glycol dimethacrylate ortriacrylates and trimethacrylates of addition products of from 6 to 20mol of ethylene oxide with 1 mol of glycerol, pentaerythritol triallylether and/or divinylurea. This invention refers to the same degree ofcrosslinking when the molar ratios between acid-functional monomersand-crosslinkers remain constant with regard to ethylenicallyunsaturated double bonds.

[0064] Possible crosslinkers also include compounds containing at leastone polymerizable ethylenically unsaturated group and at least onefurther functional group. The functional group of these crosslinkers hasto be capable of reacting with the functional groups, essentially theacid groups, of the monomers. Suitable functional groups include forexample hydroxyl, amino, epoxy and aziridino groups. Useful are forexample hydroxyalkyl esters of the abovementioned monoethylenicallyunsaturated carboxylic acids, e.g., 2-hydroxyethyl acrylate,hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethylmethacrylate, hydroxypropyl methacrylate and hydroxybutyl methacrylate,allylpiperidinium bromide, N-vinylimidazoles, for exampleN-vinylimidazole, 1-vinyl-2-methylimidazole and N-vinylimidazolines suchas N-vinylimidazoline, 1-vinyl-2-methylimidazoline,1-vinyl-2-ethylimidazoline or 1-vinyl-2-propylimidazoline, which can beused in the form of the free bases, in quaternized form or as salt inthe polymerization. It is also possible to use dialkylaminoethylacrylate and dimethylaminoethyl methacrylate, diethylaminoethyl acrylateand diethylaminoethyl methacrylate. The basic esters are preferably usedin quaternized form or as salt. It is also possible to use glycidyl(meth)acrylate, for example.

[0065] Useful crosslinkers further include compounds containing at leasttwo functional groups capable of reacting with the functional groups,essentially the acid groups, of the monomers. Suitable functional groupswere already mentioned above, i.e., hydroxyl, amino, epoxy, isocyanato,ester, amido and aziridino groups. xamples of such crosslinkers areethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, polyethylene glycol, glycerol, polyglycerol, triethanolamine,propylene glycol, polypropylene glycol, block copolymers of ethyleneoxide and propylene oxide, ethanolamine, sorbitan fatty acid esters,ethoxylated sorbitan fatty acid esters, trimethylolpropane,pentaerythritol, 1,3-butanediol, 1,4-butanediol, polyvinyl alcohol,sorbitol, starch, polyglycidyl ethers such as ethylene glycol diglycidylether, polyethylene glycol diglycidyl-ether, glycerol diglycidyl ether,glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, sorbitol polyglycidyl ether, pentaerythritolpolyglycidyl ether, propylene glycol diglycidyl ether and polypropyleneglycol diglycidyl ether, polyaziridine compounds such as2,2-bishydroxymethylbutanol tris[3-(1-aziridinyl)propionate],1,6-hexamethylenediethyleneurea,diphenylmethanebis-4,4′-N,N′-diethyleneurea, haloepoxy compounds such asepichlorohydrin and α-methylepifluorohydrin, polyisocyanates such as2,4-toluylene diisocyanate and hexamethylene diisocyanate, alkylenecarbonates such as 1,3-dioxolan-2-one and 4-methyl-1,3-dioxolan-2-one,also bisoxazolines and oxazolidones, polyamidoamines and also theirreaction products with epichlorohydrin, also polyquaternary amines suchas condensation products of dimethylamine with epichlorohydrin, homo-and copolymers of diallyldimethylammonium chloride and also homo- andcopolymers of dimethylaminoethyl (meth)acrylate which are optionallyquaternized with, for example, methyl chloride.

[0066] Useful crosslinkers further include multivalent metal ionscapable of forming ionic crosslinks. Examples of such crosslinkers aremagnesium, calcium, barium and aluminum ions. These crosslinkers areused for example as hydroxides, carbonates or bicarbonates. Usefulcrosslinkers further include multifunctional bases likewise capable offorming ionic crosslinks, for example polyamines or their quaternizedsalts. Examples of polyamines are ethylenediamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine andpolyethyleneimines and also polyamines having molar masses in each caseof up to 4 000 000.

[0067] The crosslinkers are present in the reaction mixture for examplefrom 0.001 to 20% and preferably from 0.01 to 14% by weight.

[0068] a) Free Radical Polymerization

[0069] The polymerization is initiated in the generally customarymanner, by means of an initiator. But the polymerization may also beinitiated by electron beams acting on the polymerizable aqueous mixture.However, the polymerization may also be initiated in the absence ofinitiators of the abovementioned kind, by the action of high energyradiation in the presence of photoinitiators. Useful polymerizationinitiators include all compounds which decompose into free radicalsunder the polymerization conditions, for example peroxides,hydroperoxides, hydrogen peroxides, persulfates, azo compounds and redoxcatalysts. The use of water-soluble initiators is preferred. In somecases it is advantageous to use mixtures of different polymerizationinitiators,-for example mixtures of hydrogen peroxide and sodiumperoxodisulfate or potassium peroxodisulfate. Mixtures of hydrogenperoxide and sodium peroxodisulfate may be used in any proportion.Examples of suitable organic peroxides are acetylacetone peroxide,methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumenehydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butylperneohexanoate, tert-butyl perisobutyrate, tert-butylper-2-ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate,tert-butyl perbenzoate, di(2-ethylhexyl) peroxydicarbonate, dicyclohexylperoxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate,dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allylperesters, cumyl peroxyneodecanoate, tert-butylper-3,5,5-trimethylhexanoate, acetylcyclohexylsulfonyl peroxide,dilauryl peroxide, dibenzoyl peroxide and tert-amyl perneodecanoate.Particularly suitable polymerization initiators are water-soluble azoinitiators, e.g., 2,2′-azobis(2-amidinopropane) dihydrochloride,2,2′-azobis(N,N′-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyronitrile,2,2′-azobis[2-(2′-imidazolin-2-yl)propane] dihydrochloride and4,4′-azobis(4-cyanovaleric acid). The polymerization initiatorsmentioned are used in customary amounts, for example in amounts of from0.01 to 5%, preferably from 0.05 to 2.0%, by weight, based on themonomers to be polymerized.

[0070] Useful initiators also include redox catalysts. In redoxcatalysts, the oxidizing component is at least one of theabove-specified per compounds and the reducing component is for exampleascorbic acid, glucose, sorbose, ammonium or alkali metal bisulfite,sulfite, thiosulfate, hyposulfite, pyrosulfite or sulfide, or a metalsalt, such as iron(II) ions or sodium hydroxymethylsulfoxylate. Thereducing component in the redox catalyst is preferably ascorbic acid orsodium sulfite. Based on the amount of monomers used in thepolymerization, from 3×10⁻⁶ to 1 mol % may be used for the reducingcomponent of the redox catalyst system and from 0.001 to 5.0 mol % forthe oxidizing component of the redox catalyst, for example.

[0071] When the polymerization is initiated using high energy radiation,the initiator used is customarily a photoinitiator. Photoinitiatorsinclude for example α-splitters, H-abstracting systems or else azides.Examples of such initiators are benzophenone derivatives such asMichler's ketone, phenanthrene derivatives, fluorene derivatives,anthraquinone derivatives, thioxanthone derivatives, coumarinderivatives, benzoin ethers and derivatives thereof, azo compounds suchas the abovementioned free-radical formers, substitutedhexaarylbisimidazoles or acylphosphine oxides. Examples of azides are:2-(N,N-dimethylamino)ethyl 4-azidocinnamate, 2-(N,N-dimethylamino)ethyl4-azidonaphthyl ketone, 2-(N,N-dimethylamino)ethyl 4-azidobenzoate,5-azido-1-naphthyl 2′-(N,N-dimethylamino)ethyl sulfone,N-(4-sulfonylazidophenyl)maleimide, N-acetyl-4-sulfonylazidoaniline,4-sulfonylazidoaniline, 4-azidoaniline, 4-azidophenacyl bromide,p-azidobenzoic acid, 2,6-bis(p-azidobenzylidene)cyclohexanone and2,6-bis(p-azidobenzylidene)-4-methylcyclohexanone. Photoinitiators, ifused, are customarily used in amounts of from 0.01 to 5% of the weightof the monomers to be polymerized.

[0072] The crosslinked polymers are preferably used in partiallyneutralized form. The degree of neutralization is preferably in therange from 5 to 60 mol %, more preferably in the range from 10 to 40 mol%, particularly preferably in the range from 20 to 30 mol %, based onthe monomers containing acid groups. Useful neutralizing agents includealkali metal bases or ammonia/amines. Preference is given to the use ofaqueous sodium hydroxide solution, aqueous potassium hydroxide solutionor lithium hydroxide. However, neutralization may also be effected usingsodium carbonate, sodium bicarbonate, potassium carbonate or potassiumbicarbonate or other carbonates or bicarbonates or ammonia. Moreoverprimary, secondary and tertiary amines may be used.

[0073] Alternatively, the degree of neutralization can be set before,during or after polymerization in all apparatuses suitable for thispurpose. Neutralization can be effected for example directly in akneader used for polymerization. Neutralization is preferably carriedout before polymerization. Industrial processes useful for making theseproducts include all processes which are customarily used to makesuperabsorbers, as described for example in Chapter 3 of “ModernSuperabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham,Wiley-VCH, 1998.

[0074] Polymerization in aqueous solution is preferably conducted as agel polymerization. It involves 10-70% strength by weight aqueoussolutions of the monomers and optionally of a suitable grafting basebeing polymerized in the presence of a free-radical initiator byutilizing the Trommsdorff-Norrish effect.

[0075] The polymerization reaction may be carried out at from 0 to 150°C., preferably at from 10 to 100° C., not only at atmospheric pressurebut also at superatmospheric or reduced pressure. As is customary, thepolymerization may also be conducted in a protective gas atmosphere,preferably under nitrogen.

[0076] By subsequently heating the polymer gels at from 50 to 130° C.,preferably at from 70 to 100° C., for several hours, the performancecharacteristics of the polymers can be further improved.

[0077] The following list relates to particularly preferred conditionsfor production processes of gels and of dried gels prior to surfacepostcrosslinking (base polymers). The missing weight percent from 100%are to be made up with water.

[0078] Base Polymer 1:

[0079] Acrylic acid: 25-40% by weight based on batch size, preferably28-35% by weight, especially about 31% by weight based on batch size;

[0080] NaOH 50%: 22-30 mol % based on acrylic acid, preferably 22-29 mol%, more preferably from 23 to 28 mol %, particularly preferably from 24to 27 mol %, especially about 25.2 mol % based on acrylic acid, acorresponding neutralization can also be achieved with otherneutralizing agents;

[0081] polyethylene glycol 400 diacrylate: 0.005-1.0% by weight based onacrylic acid, preferably 0.1-0.4% by weight, particularly preferably0.15-0.25% by weight, especially about 0.2% by weight based on acrylicacid;

[0082] sodium persulfate: 0.2-0.4% by weight based on acrylic acid,preferably 0.25-0.35% by weight, especially about 0.28% by weight basedon acrylic acid;

[0083] ascorbic acid: 0.005-0.006% by weight based on acrylic acid,preferably 0.0053-0.0058% by weight especially about 0.0056% by weightbased on acrylic acid;

[0084] sorbitan monococoate: from 0 to 0.1% by weight based on acrylicacid;

[0085] useful crosslinkers further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds, for example ETMPTA(ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X fromBASF AG) in place of the polyethylene glycol 400 diacrylate): 0.2-0.5%by weight based on acrylic acid; what is decisive is that approximatelythe same degree of crosslinking is achieved.

[0086] Useful initiators further include other similarly acting systemsor individual components. In the case of radiative initiation,appropriate radiation initiators have to be used.

[0087] Temperature at polymerization start: 15-35° C., especially about25° C.;

[0088] The resulting gel 1 is converted by drying into the base polymer1.

[0089] Drying temperature of gel: 120-200° C., preferably 140-180° C.,especially about 160° C.

[0090] Base Polymer 2:

[0091] Acrylic acid: 25-40% by weight based on batch size, preferably28-35% by weight, especially about 30% by weight based on batch size;

[0092] NaHCO₃ 23-38% by weight based on acrylic acid, preferably from 27to 35% by weight, particular preferably from 30 to 33% by weight,especially about 31.5% by weight based on acrylic acid, a correspondingneutralization can also be achieved with other neutralizing agents;

[0093] sorbitan monococoate: from 0 to 0.15% by weight based on acrylicacid, preferably 0.02-0.1% by weight, especially about 0.065% by weightbased on acrylic acid;

[0094] allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,preferably 0.1-0.5% by weight, particularly preferably 0.2-0.4% byweight, especially about 0.3% by weight based on acrylic acid;

[0095] 2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight basedon acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%by weight based on acrylic acid;

[0096] potassium peroxodisulfate: 0.01-0.3% by weight based on acrylicacid, preferably 0.1-0.2% by weight, especially about 0.167% by weightbased on acrylic acid;

[0097] ascorbic acid: 0.005-0.03% by weight based on acrylic acid,preferably 0.01-0.02% by weight especially about 0.015% by weight basedon acrylic acid;

[0098] useful crosslinkers further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds, for example ETMPTA(ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X fromBASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weightbased on acrylic acid; what is decisive is that approximately the samedegree of crosslinking is achieved.

[0099] Useful initiators further include other similarly acting systemsor individual components. In the case of radiative initiation,appropriate radiation initiators have to be used.

[0100] Maximum temperature during polymerization: 70-100° C., especiallyabout 90° C.;

[0101] The resulting gel 2 is converted by drying into the base polymer2.

[0102] Drying temperature of gel: 120-200° C., preferably about 130-150°C., especially about 135° C.

[0103] Base Polymer 3:

[0104] Acrylic acid: 25-40% by weight based on batch size, preferably28-35% by weight, especially about 30% by weight based on batch size;

[0105] LiOH×H₂O: 15-30% by weight based on acrylic acid, preferably from17 to 25% by weight, particular preferably from 19 to 23% by weight,epecially about 20.4% by weight based on acrylic acid, a correspondingneutralization can also be achieved with other neutralizing agents;

[0106] sorbitan monococoate: from 0 to 0.15% by weight based on acrylicacid, preferably 0.02-0.1% by weight, especially about 0.065% by weightbased on acrylic acid;

[0107] allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% byweight, especially about 0.4% by weight based on acrylic acid;

[0108] 2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight basedon acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%by weight based on acrylic acid;

[0109] potassium peroxodisulfate: 0.01-0.3% by weight based on acrylicacid, preferably 0.1-0.2% by weight, especially about 0.167% by weightbased on acrylic acid;

[0110] ascorbic acid: 0.005-0.03% by weight based on acrylic acid,preferably 0.01-0.02% by.weight especially about 0.015% by weight basedon acrylic acid;

[0111] useful crosslinkers further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds, for example ETMPTA(ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X fromBASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weightbased on acrylic acid; what is decisive is that approximately the samedegree of crosslinking is achieved.

[0112] Useful initiators further include other similarly acting systemsor individual components. In the case of radiative initiation,appropriate radiation initiators have to be used.

[0113] Maximum temperature during polymerization: 70-100° C., especiallyabout 90° C.;

[0114] The resulting gel 3 is converted by drying into the base polymer3.

[0115] Drying temperature of gel: 120-200° C., preferably about 130-150°C., especially about 140° C.

[0116] Base Polymer 4:

[0117] Acrylic acid: 15-28% by weight based on batch size, preferably18-24% by weight, especially about 21% by weight based on batch size;

[0118] NaHCO₃ 35-55% by weight based on acrylic acid, preferably from 40to 50% by weight, particular preferably from 43 to 46% by weight,especially about 44.5% by weight based on acrylic acid, a correspondingneutralization can also be achieved with other neutralizing agents;

[0119] 2-acrylamido-2-methylpropanesulfonic acid: 30-55% by weight basedon acrylic acid, preferably from 35 to 50% by weight, particularlypreferably from 41 to 45% by weight, especially about 43% by weightbased on acrylic acid;

[0120] sorbitan monococoate: from 0 to 0.15% by weight based on acrylicacid, preferably 0.02-0.1% by weight, especially about 0.065% by weightbased on acrylic acid;

[0121] allyl methacrylate: 0.005-1.0% by weight based on acrylic acid,preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% byweight, especially about 0.4% by weight based on acrylic acid;

[0122] 2,2′-azobisamidinopropane dihydrochloride: 0-0.2% by weight basedon acrylic acid, preferably 0.05-0.1% by weight, especially about 0.08%by weight based on acrylic acid;

[0123] potassium peroxodisulfate: 0.01-0.3% by weight based on acrylicacid, preferably 0.1-0.2% by weight, especially about 0.167% by weightbased on acrylic acid;

[0124] ascorbic acid: 0.005-0.03% by weight based on acrylic acid,preferably 0.01-0.02% by weight especially about 0.015% by weight basedon acrylic acid;

[0125] useful crosslinkers further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds, for example ETMPTA(ethoxylated trimethylolpropane triacrylate (LAROMER® LR 9015 X fromBASF AG) or polyethylene glycol 400 diacrylate): 0.2-0.5% by weightbased on acrylic acid; what is decisive is that approximately the samedegree of crosslinking is achieved.

[0126] Useful initiators further include other similarly acting systemsor individual components. In the case of radiative initiation,appropriate radiation initiators have to be used.

[0127] Maximum temperature during polymerization: 70-100° C., especiallyabout 90° C.;

[0128] The resulting gel 4 is converted by drying into the base polymer4.

[0129] Drying temperature of gel: 120-200° C., preferably about 130-150°C., especially about 140° C.

[0130] Base Polymer 5:

[0131] Acrylic acid: 20-35% by weight based on-batch size, preferably24-28% by weight, especially about 26% by weight based on batch size;

[0132] NaOH 50%: 30-60 mol % based on acrylic acid, preferably from 40to 55 mol %, particular preferably from 44 to 51 mol %, especially about45 or 50-mol % based on acrylic acid, a corresponding neutralization canalso be achieved with other neutralizing agents;

[0133] ETMPTA: 0.005-1.0% by weight on acrylic acid, preferably0.01-0.4% by weight, particularly preferably 0.03-0.2% by weight,especially about 0.06% by weight based on acrylic acid

[0134] sodium persulfate: 0.2-0.4% by weight based on acrylic acid,preferably 0.25-0.35% by weight, especially about 0.28% by weight basedon acrylic acid;

[0135] photoinitiators Darocur 1173: Irgacure 651 (2:1 ratio):0.005-0.1% by weight based on acrylic acid, preferably 0.01-0.05% byweight especially about 0.024% by weight based on acrylic acid;

[0136] useful crosslinkers further include other crosslinkers having atleast 2 ethylenically unsaturated double bonds; what is decisive is thatapproximately the same degree of crosslinking is achieved.

[0137] Useful initiators further include other similarly acting systemsor individual components, the above-indicated system preferably beinginitiated using a UV lamp. In the case of thermal initiation,appropriate initiator systems have to be used.

[0138] The gel obtained may before drying be optionally additionallytreated with 0-0.1% by weight, preferably 0.01-0.05% by weightespecially about 0.026% by weight of sodium metabisulphite and with0-0.1% by weight, preferably 0.01-0.05% by weight, especially about0.02% by weight of sorbitan monolaurate.

[0139] The resulting gel 5 is converted by drying into the base polymer5.

[0140] Drying temperature of gel: 120-200° C., preferably about 130-150°C., especially about 145° C.

[0141] Base Polymer 6:

[0142] Acrylic acid: 25-40% by weight based on batch size, preferably28-35% by weight, especially about 30% by weight based on batch size;

[0143] NaOH 50%: 30-60 mol % based on acrylic acid, preferably from 40to 55 mol %, particularly preferably from 48 to 52 mol %, especiallyabout 50 mol % based on acrylic acid, a corresponding neutralization canalso be achieved with other neutralizing agents;

[0144] polyethylene glycol 400 diacrylate: 0.005-1.1% by weight based onacrylic acid, preferably 0.1-1.0% by weight, particularly preferably0.3-0.6% by weight, especially about 0.45% by weight based on acrylicacid;

[0145] sodium persulfate: 0.2-0.4% by weight based on acrylic acid,preferably 0.25-0.35% by weight,-especially about 0.29% by weight basedon acrylic acid;

[0146] ascorbic acid: 0.005-0.01% by weight based on acrylic acid, 40preferably 0.006-0.008% by weight especially about 0.007% by weightbased on acrylic acid;

[0147] sorbitan monococoate: from 0 up to 0.1% by weight based onacrylic acid; especially about 0.08% by weight based on acrylic acid.

[0148] Useful crosslinking agents further include other crosslinkershaving at least 2 ethylenically unsaturated double bonds, such as forexample ETMPTA (ethoxylated trimethylolpropane triacrylate (LAROMER® LR9015 X from BASF AG) in place of the polyethylene glycol 400diacrylate): 0.2-0.5% by weight based on acrylic acid; what is decisiveis that approximately the same degree of crosslinking is achieved.

[0149] Useful initiators further include other similarly acting systemsor individual components. In the case of radiative initiation,appropriate radiation initiators have to be used.

[0150] Temperature at polymerization start: 15-35° C., especially about25° C.;

[0151] The resulting gel 6 is converted by drying into the base polymer6.

[0152] Drying temperature of gel: 120-200° C., preferably 140-180 C,especially about 160° C.

[0153] Not only the gels 1-6 but also the base polymers 1-6 areimportant intermediates for the preparation of the inventive hydrogelforming polymers capable of absorbing aqueous fluids. The gels and basepolymers mentioned in the claims are preferred.

[0154] Especial preference is given to a gel prepared by polymerizationof partially neutralized acrylic acid with a crosslinking agent, wherein

[0155] (i) the partial neutralization can be effected using NaOH 50%:

[0156] 20-30 mol % based on acrylic acid, preferably 22-29 mol %, morepreferably from 23 to 28 mol %, particularly preferably from 24 to 27mol %, especially about 25.2 mol % based on acrylic acid, acorresponding neutralization can also be achieved with otherneutralizing agents,

[0157] the crosslinking can be effected using polyethylene glycol 400diacrylate: 0.005-1.0% by weight based on acrylic acid, preferably0.1-0.4% by weight, particularly preferably 0.15-0.25% by weight,especially about 0.2% by weight based on acrylic acid;

[0158] or some other crosslinker which produces the same degree ofcrosslinking,

[0159] or

[0160] (ii)the partial neutralization can be effected using NaHCO₃:

[0161] 23-28% by weight based on acrylic acid, preferably from 27 to 35%by weight, particularly preferably from 30 to 33% by weight, especiallyabout 31.5% by weight based on acrylic acid, a correspondingneutralization can also be achieved with other neutralizing agents,

[0162] the crosslinking can be effected using allyl methacrylate:0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.5% byweight, particularly preferably 0.2-0.4% by weight, especially about0.3% by weight based on acrylic acid;

[0163] or some other crosslinker which produces the same degree ofcrosslinking,

[0164] or

[0165] (iii) the partial neutralization can be effected using LiOH×H₂O:

[0166] 15-30% by weight based on acrylic-acid, preferably from 17 to 25%by weight, particularly preferably from 19 to 23% by weight, especiallyabout 20.4% by weight based on acrylic acid, a correspondingneutralization can also be achieved with other neutralizing agents,

[0167] the crosslinking can be effected using allyl methacrylate:0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6% byweight, particularly preferably 0.3-0.5% by weight, especially about0.4% by weight based on acrylic acid;

[0168] or some other crosslinker which produces the same degree ofcrosslinking,

[0169] or

[0170] (iv)the partial neutralization can be effected using NaHCO₃:

[0171] 35-55% by weight based on acrylic acid, preferably from 40 to 50%by weight, particularly preferably from 43 to 46% by weight, especiallyabout 44.5% by weight based on acrylic acid, a correspondingneutralization can also be achieved using other neutralizing agents when

[0172] 2-acrylamido-2-methylpropanesulfonic acid is present at 30-55% byweight based on acrylic acid, preferably from 35 to 50% by weight,particularly preferably from 41 to 45% by weight, especially about 43%by weight based on acrylic acid additionally to acrylic acid,

[0173] the crosslinking can be effected using allyl methacrylate:0.005-1.0% by weight based on acrylic acid, preferably 0.1-0.6% byweight, particularly preferably 0.3-0.5% by weight, especially about0.4% by weight based on acrylic acid,

[0174] or some other crosslinker which produces the same degree of crosslinking

[0175] or

[0176] (v) the partial neutralization can be effected using NaOH 50%:

[0177] 30-60 mol % based on acrylic acid, preferably from 40 to 55 mol%, particularly preferably from 45 to 51 mol %, especially about 45 or50 mol % based on acrylic acid, a corresponding neutralization can alsobe achieved with other neutralizing agents,

[0178] the crosslinking can be effected using ETMPTA: 0.005-1.0% byweight based on acrylic acid, preferably 0.1-0.4% by weight,particularly preferably 0.03-0.2% by weight, especially about 0.06% byweight based on acrylic acid; or some other crosslinker which producesthe same degree of crosslinker,

[0179] or

[0180] (vi)the partial neutralization can be effected using NaOH 50%:

[0181] 30-60 mol % based on acrylic acid, preferably from 40 to 55 mol%, particularly preferably from 48 to 52 mol %, especially about 50 mol% based on acrylic acid, a corresponding neutralization can also beachieved with other neutralizing agents,

[0182] the crosslinking can be effected using polyethylene glycol 400diacrylate: 0.005-1.1% by weight based on acrylic acid, preferably0.1-1.0% by weight, particularly preferably 0.3-0.6% by weight,especially about 45% by weight, based on acrylic acid, or some othercrosslinker which produces the same degree of crosslinking.

[0183] The abovementioned gels are converted by drying at 120° C.-200°C. into the base polymers according to the present invention. The watercontent is then preferably below 5% by weight and especially below 3% byweight. The base polymers can be converted y the hereinbelow preferredsurface postcrosslinking conditions into the correspondingsuperabsorbents, the conditions indicated for each base polymer beingpreferred for the specific base polymer.

[0184] a) Surface Postcrosslinking

[0185] Preference is given to hydrogel forming polymers which have beensurface-postcrosslinked. Surface postcrosslinking may be carried out ina conventional manner using dried, ground and classified polymerparticles.

[0186] To effect surface postcrosslinking, compounds capable of reactingwith the functional groups of the polymers by crosslinking are appliedto the surface of the hydrogel particles, preferably in the form of anaqueous solution. The aqueous solution may contain water-miscibleorganic solvents. Suitable solvents are alcohols such as methanol,ethanol, i-propanol ethylene glycol, propylene glycol or acetone.

[0187] The subsequent crosslinking reacts polymers which have beenprepared by the polymerization of the abovementioned monoethylenicallyunsaturated acids and optionally monoethylenically unsaturatedcomonomers and which have a molecular weight of greater than 5 000,preferably greater than 50 000, with compounds which have at least twogroups reactive toward acid groups. This reaction can take place at roomtemperature or else at elevated temperatures up to 220° C.

[0188] Suitable postcrosslinkers include for example:

[0189] di- or polyglycidyl compounds such as diglycidyl phosphonates orethylene glycol diglycidyl ether, bischlorohydrin ethers of polyalkyleneglycols,

[0190] alkoxysilyl compounds,

[0191] polyaziridines, aziridine compounds based on polyethers orsubstituted hydrocarbons, for example bis-N-aziridinomethane,

[0192] polyamines or polyamidoamines and their reaction products withepichlorohydrin,

[0193] polyols such as ethylene glycol, 1,2-propanediol, 1,4-butanediol,glycerol, methyltriglycol, polyethylene glycols having an averagemolecular weight M_(w) of 200-10 000, di- and polyglycerol,pentaerythritol, sorbitol, the ethoxylates of these polyols and theiresters with carboxylic acids or carbonic acid such as ethylene carbonateor propylene carbonate,

[0194] carbonic acid derivatives such as urea, thiourea, guanidine,dicyandiamide, 2-oxazolidinone and its derivatives, bisoxazoline,polyoxazolines, di- and polyisocyanates,

[0195] di- and poly-N-methylol compounds such as, for example,methylenebis(N-methylolmethacrylamide) or melamine-formaldehyde resins,

[0196] compounds having two or more blocked isocyanate groups such as,for example, trimethylhexamethylene diisocyanate blocked with2,2,3,.6-tetramethylpiperidin-4-one.

[0197] If necessary, acidic catalysts may be added, for examplep-toluenesulfonic acid, phosphoric acid, boric acid or ammoniumdihydrogenphosphate.

[0198] Particularly suitable postcrosslinkers are di- or polyglycidylcompounds such as ethylene glycol diglycidyl ether, the reactionproducts of polyamidoamines with epichlorohydrin and 2-oxazolidinone.

[0199] The crosslinker solution is preferably applied by spraying with asolution of the crosslinker in conventional reaction mixers or mixingand drying equipment such as Patterson-Kelly mixers, DRAIS turbulencemixers, Lödige mixers, screw mixers, plate mixers, fluidized bed mixersand Schugi Mix. The spraying of the crosslinker solution may be followedby a heat treatment step, preferably in a downstream dryer, at from 80to 230° C., preferably 80-190° C., particularly preferably at from 100to 160° C., for from 5 minutes to 6 hours, preferably from 10 minutes to2 hours, particularly preferably from 10 minutes to 1 hour, during whichnot only cracking products but also solvent fractions can be removed.But the drying may also take place in the mixer itself, by heating thejacket or-by blowing in a preheated carrier gas.

[0200] In a particularly preferred embodiment of the invention, thehydrophilicity of the particle surface of the hydrogel-forming polymeris additionally modified by formation of complexes. The formation ofcomplexes on the outer shell of the hydrogel particles is effected byspraying with solutions of divalent or more highly valent metal saltsolutions, and the metal cations can react with the acid groups of thepolymer to form complexes. Examples of divalent or more highly valentmetal cations are Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe²⁺/³⁺, Co²⁺,Ni²⁺, Cu⁺/²⁺, Zn²⁺, Y³⁺, Zr⁴⁺, Ag⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au⁺/³⁺,preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺ and La³⁺, andparticularly preferred metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. The metalcations may be used not only alone but also mixed with each other. Ofthe metal cations mentioned, all metal salts are suitable that possessadequate solubility in the solvent to be used. Of particular suitabilityare metal salts with weakly complexing anions such as for examplechloride, nitrate and sulfate. Useful solvents for the metal saltsinclude water, alcohols, DMF, DMSO and also mixtures thereof. Particularpreference is given to water and water-alcohol mixtures such as forexample water-methanol or water-1,2-propanediol.

[0201] The spraying of the metal salt solution onto the particles of thehydrogel-forming polymer may be effected not only before but also afterthe surface postcrosslinking of the particles. In a particularlypreferred process, the spraying of the metal salt solution takes placein the same step as the spraying with the crosslinker solution, the twosolutions being sprayed separately in succession or simultaneously viatwo nozzles or the crosslinker and metal salt solutions may be sprayedconjointly through a single nozzle.

[0202] Optionally, the hydrogel-forming polymers may be further modifiedby admixture of finely divided inorganic solids, for example silica,alumina, titanium dioxide and iron(II) oxide, to further augment theeffects of the surface aftertreatment. Particular preference is given tothe admixture of hydrophilic silica or of alumina having an averageprimary particle size of from 4 to 50 nm and a specific surface area of50-450 m²/g. The admixture of finely divided inorganic solids preferablytakes place after the surface modification throughcrosslinking/complexing, but may also be carried out before or duringthese surface modifications.

[0203] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 1:

[0204] ethylene glycol diglycidyl ether: 0.01-0.12% by weight based ondried gel (base polymer), preferably 0.012-0.02% by weight, especiallyabout 0.015% by weight based on base polymer;

[0205] water: 1.5-5% by weight based on base polymer, preferably 1.6-2%by weight, especially about 1.67% by weight based on base polymer;

[0206] sorbitan monococoate: 0-0.1% by weight based on base polymer,preferably 0.03-0.07% by weight, especially about 0.05% by weight basedon base polymer;

[0207] heat treatment jacket temperature: 120-180° C, preferably140-160° C., especially 150° C.; heat treatment residence time has to beconformed to the temperature, higher temperatures involving shorterresidence times and longer residence times giving rise to morepronounced postcrosslinking. Typical values are 150-10 minutes,especially about 120 minutes.

[0208] ethylene glycol diglycidyl ether: 0.01-0.12% by weight based ondried gel (base polymer), preferably 0.08-0.11% by weight, especiallyabout 0.10% by weight based on base polymer;

[0209] water: 0.5-5% by weight based on base polymer, preferably 1-2% byweight, especially about 1.5% by weight based on base polymer;

[0210] 1,2-propanediol: 0-3.5% by weight based on base polymer,preferably 0.5-1.5% by weight, especially about 0.8% by weight based onbase polymer;

[0211] aluminum sulfate (as 26.8% solution for example): 0-0.15% byweight based on base polymer, preferably 0.03-0.10% by weight,especially about 0.075% by weight based on base polymer;

[0212] heat treatment jacket temperature: 120-180° C., preferably140-160° C., especially 150° C.; heat treatment residence time has to beconformed to the temperature, higher temperatures involving shorterresidence times and longer residence times giving rise to morepronounced postcrosslinking. Typical values are 150-10 minutes.,especially about 15 minutes.

[0213] ethylene glycol diglycidyl ether: 0.01-0.12% by weight based ondried gel (base polymer), preferably 0.08-0.11% by weight, especiallyabout 0.10% by weight based on base polymer;

[0214] water: 0.5-5% by weight based on base polymer, preferably 1-2% byweight, especially about 1.67% by weight based on base polymer;

[0215] heat treatment jacket temperature: 120-180° C., preferably140-160° C., especially 150° C.; heat treatment residence time has to beconformed to the temperature, higher temperatures involving shorterresidence times and longer residence times giving rise to morepronounced postcrosslinking. Typical values are 150-10 minutes,especially about 15 minutes.

[0216] The longer the postcrosslinking time, the higher the resultantAUL, although CRC decreases somewhat.

[0217] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 2:

[0218] oxazolidinone: 0.005-0.1% by weight based on dried gel (basepolymer), preferably 0.01-0.05% by weight, especially about 0.025% byweight based on-base polymer;

[0219] water: 0.5-5% by weight based on base polymer, preferably 1-3% byweight, especially about 2% by weight based on base polymer;

[0220] 1,2-propanediol: 0-4% by weight based on base polymer, preferably0.5-3% by weight, especially about 2% by weight based on base polymer;

[0221] aluminum sulfate×8H₂O: 0-0.3% by weight based on base polymer,preferably 0.03-0.10% by weight, especially about 0.05% by weight basedon base polymer;

[0222] heat treatment jacket temperature: 120-180° C., preferably140-160° C., especially 150° C.; heat treatment residence time has to beconformed to the temperature, higher temperatures involving shorterresidence times and longer residence times giving rise to morepronounced postcrosslinking. Typical values are 150-10 minutes,preferably 90-20 minutes, especially about 30, 60 or 70 minutes.

[0223] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 3:

[0224] ethylene glycol diglycidyl ether: 0.005-0.12% by weight based ondried gel (base polymer), preferably 0.01-0.05% by weight, especiallyabout 0.03% by weight based on base polymer;

[0225] water: 0.5-5% by weight based on base polymer, preferably 2-4% byweight, especially about 3.35% by weight based on base polymer;

[0226] 1,2-propanediol: 0-4% by weight based on base polymer, preferably0.5-3% by weight, especially about 1.6% by weight based on base polymer;

[0227] aluminum sulfate: 0-0.3% by weight based on base polymer,preferably 0.05-0.10% by weight, especially about 0.075% by weight basedon base polymer;

[0228] heat treatment circulating air drying cabinet temperature:80-180° C., preferably 90-120° C., especially about 100° C.; heattreatment residence time has to be conformed to the temperature, highertemperatures involving shorter residence times and longer residencetimes giving rise to more pronounced postcrosslinking. Typical valuesare 180-20 minutes, preferably 150-90 minutes, especially about 120minutes.

[0229] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 4:

[0230] ethylene glycol diglycidyl ether: 0.005-0.3% by weight based ondried gel (base polymer), preferably 0.05-0.1% by weight, especiallyabout 0.075% by weight based on base polymer or preferably 0.1-0.3% byweight, especially about 0.2% by weight based on base polymer;

[0231] water: 0.5-5% by weight based on base polymer, preferably 1-3% byweight, especially about 2% by weight or 2.3% by weight based on basepolymer;

[0232] 1,2-propanediol: 0-4% by weight based on base polymer, preferably1-3% by weight, especially about 2% by weight or 1.2% by weight based onbase polymer;

[0233] aluminum sulfate: 0-0.3% by weight based on base polymer,preferably 0.05-0.10% by weight, especially about 0.075% by weight basedon base polymer;

[0234] heat treatment circulating air drying cabinet temperature:80-180° C., preferably 100-160° C., especially about 140° C.; heattreatment residence time has to be conformed to the temperature, highertemperatures involving shorter residence times and longer residencetimes giving rise to more pronounced postcrosslinking. Typical valuesare 180-20 minutes, preferably 150-90 minutes, especially about 120minutes.

[0235] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 5:

[0236] ethylene glycol diglycidyl ether: 0.005-0.2% by weight based ondried gel (base polymer), preferably 0.03-0.1% by weight, especiallyabout 0.06% by weight based on base polymer;

[0237] water: 0.5-5% by weight based on base polymer, preferably .1-3%by weight, especially about 2.5% by weight based on base polymer;

[0238] 1,2-propanediol: 0-4% by weight based on base polymer, preferably0.5-3% by weight, especially about 1.5% by weight based on base polymer;

[0239] heat treatment circulating air drying cabinet temperature:80-180° C., preferably 100-160° C., especially about 145° C.; heattreatment residence time has to be conformed to the temperature, highertemperatures involving shorter residence times and longer residencetimes giving rise to more pronounced postcrosslinking. Typical valuesare 180-20 minutes, preferably 90-30 minutes, especially about 60minutes.

[0240] The following list relates to particularly preferredpostcrosslinking conditions of dried gels according to the invention,especially to particularly preferred postcrosslinking conditions of basepolymer 6:

[0241] ethylene glycol diglycidyl ether: 0.01-0.12% by weight based ondried gel (base polymer), preferably 0.04-0.08% by weight, especiallyabout 0.06% by weight based on base polymer;

[0242] water: 0.5-5% by weight based on base polymer, preferably 2-4% byweight, especially about 3.2% by weight based on base polymer;

[0243] 1,2-propanediol: 0-4% by weight based on base polymer, preferably2% by weight, especially about 1.6% by weight based on base polymer 2;

[0244] heat treatment circulating air drying cabinet temperature:120-180° C., preferably 140-160° C., especially 150° C.; heat treatmentresidence time has to be conformed to the temperature, highertemperatures involving shorter residence times and longer residencetimes giving rise to more pronounced postcrosslinking. Typical valuesare 150-10 minutes, especially about 120 minutes.

[0245] The longer the postcrosslinking time, the higher the resultantAUL, although CRC decreases somewhat.

[0246] Properties of acidic hydrogel forming polymers according to theinvention.

[0247] The inventive acidic hydrogel forming polymers capable ofabsorbing aqueous fluids have a particle size distribution which isgenerally in the range from 10 μm to about 1000 μm, preferably in therange from about 100 μm to about 850 μm and especially in the range from150 μm to about 700 μm. The size windows mentioned preferably includemore than 80% by weight and especially more than 90% by weight of theparticles.

[0248] The inventive acidic hydrogel forming polymers capable ofabsorbing aqueous fluids comprise improved odor control properties aswell as high ultimate absorption capacity, high gel strength andpermeability and also high retention. Owing to the presence of acidichydrogel forming polymers, the products of the invention haveantimicrobial properties, thereby providing an odor control systemwithout the need for the addition of odor inhibiting substances or odormasking materials.

[0249] In contrast to the prior art, where an added odor control unit isindispensable for the use of superabsorbent polymers in the hygienesector, the products of the invention permit substantially less costlymanufacture, since as well as there being no need for an odor controlunit there is no need either for binders or other aids for binding anodor control unit to hydrogel forming polymers.

[0250] The reduction or preferably the elimination of additives for odorcontrol purposes results in no changes to the high absorptionperformance and no changes to the excellent absorption behavior of thehydrogel forming polymer material used. This in turn provides longerwear times when the products of the invention are used in a hygienearticle. Skin sensitization and irritation is completely avoided andeliminated by a constant pH medium. The pH of the hydrogel formingpolymers according to the invention can be measured by the methodsindicated in the description part and is 5.7 or less, especially 5.6,5.5, 5.4, 5.3, 5.2 or 5.1 and less, preferably 5.0 especially 4.9, 4.8,4.7 4.6 and less, particularly preferably 4.5; the lower limit isparticularly preferably 4.4 especially 4.3, 4.2 or 4.1, preferably 4.0especially 3.9, 3.8, 3.7, 3.6, 3.5, 3.4, 3.3, 3.2, 3.1 or 3.0,preference being given to combinations of the upper and lower limits,for example pH values in the range from 3 to 5.7, preferably in therange from 4 to 5.5 and particularly preferably in the range from 4.4 to4.6 or from 5.1 to 5.3.

[0251] The SFC value [in 10⁻⁷ cm³s/g] of the hydrogel forming polymersaccording to the invention can be measured by the methods indicated inthe description part and is preferably above 1, especially 2, 4, 6, 8,10, 12, 14, 16, 18, 20 or higher, particularly preferably 22, especially24, 26, 28, 30, 32 or higher.

[0252] The CRC value [g/g] of the hydrogel forming polymers according tothe invention can be measured by the methods indicated in thedescription part and is preferably above 15, especially 16, 18, 20, 22,24, or higher, particularly preferably 25, especially 26, 27, 28, 29,30, 31, 32 or higher.

[0253] The AUL-0.7 psi value [g/g] of the hydrogel forming polymersaccording to the invention can be measured by the methods indicated inthe description part and is preferably above 4, especially 6, 8, 10, 12,or higher, particularly preferably 13, especially 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, or higher.

[0254] The PH_(AI) value of the hydrogel forming polymers according tothe invention can be measured and calculated by the methods indicated inthe description part and is at least 80 or higher, especially 81, 82,83, 84, 85, 86, 87, 88, 90 or higher, preferably above 91, especially92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107,108, 109, 110, 111, 112, 113, 114, 115, 116, 117 118, 119, 120 orhigher.

[0255] The Nessler value (measured as N₂ from NH₃ in mg/l compared withN₂ from NH₃ of Example 9. The N₂ value of Example 9 is set at 100%. Thehydrogel forming polymer according to the invention can be measured andcalculated by the methods indicated in the description part and is atmost 65% or less, especially 60%, 55%, 50% of the value of HySorb C7015® or less, preferably less than 45%, especially 40%, 39%, 38%, 37%,36%, 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%,22%, 21%, 20% or less.

[0256] Particular preference is given to a combination of the thresholdvalues, for example PHAI with pH, PHAI with SFC, PHAI with CRC, PH_(AI)with AUL, PHAI with Nessler, especially triple combinations such asPH_(AI) with pH and SFC, PH_(AI) with pH and CRC, PH_(AI) with pH andAUL, PH_(AI) with Nessler and SFC, PH_(AI) with Nessler and CRC, PH_(AI)with Nessler and AUL.

[0257] Use of Acidic Hydrogel Forming Polymers

[0258] The present invention further provides for the use of theabovementioned hydrogel forming polymers in hygiene articles comprising

[0259] (A) a liquid pervious topsheet

[0260] (B) a liquid impervious backsheet.

[0261] (C) a core positioned between (A) and (B) and comprising

[0262] 10-100% by weight of the hydrogel forming polymer according tothe invention

[0263] 0-90% by weight of hydrophilic fiber material

[0264] preferably 20-100% by weight of the inventive hydrogel formingpolymer, 0-80.% by weight of hydrophilic fiber material

[0265] more preferably 30-100% by weight of the inventive hydrogelforming polymer, 0-70% by weight of hydrophilic fiber material

[0266] even more preferably 40-100% by weight of the inventive hydrogelforming polymer, 0-60% by weight of hydrophilic fiber material

[0267] much more preferably 50-100% by weight of the inventive hydrogelforming polymer, 0-50% by weight of hydrophilic fiber material

[0268] particularly preferably 60-100% by weight of the inventiveforming polymer, 0-40% by weight of the hydrophilic fiber material

[0269] especially preferably 70-100% by weight of the inventive hydrogelforming polymer, 0-30% by weight of the hydrophilic fiber material

[0270] extremely preferably 80-100% by weight of.the inventive hydrogelforming polymer, 0-20% by weight of the hydrophilic fiber material

[0271] most preferably 90-100% by weight of the inventive hydrogelforming polymer, 0-10% by weight of the hydrophilic fiber material

[0272] (D) optionally a tissue layer positioned directly above and belowsaid core (C) and

[0273] (E) optionally an acquisition layer positioned between (A) and(C).

[0274] The percentages are to be understood so that in the case of10-100% by weight 11, 12, 13, 14, 15, 16, 17, 18, 19 up to in each case100% by weight of hydrogel forming polymer according to the inventionand all in between %ages (for example 12.2%) are possible andcorrespondingly hydrophilic fiber material from 0 to respectively 89,88, 87, 86, 85, 83, 82, 81% by weight and in between percentages (forexample 87.8%) are possible. When further materials are present in thecore, then the percentages of polymer and fiber are reduced accordingly.The same applies to the preferred ranges, for example in the case ofextremely preferably 81, 82, 83, 84, 85, 86, 87, 88, 89% by weight canbe present for the hydrogel forming polymer of the invention andcorrespondingly 19,. 18, 17, 16, 15, 14, 13, 12, 11% by weight of thefiber material. So the preferred range contains 20, 21, 22, 23, 24, 25,26, 27, 28, 29 to 100% by weight of hydrogel forming polymer accordingto the invention, the more preferred range 30, 25 31, 32, 33, 34, 35,36, 37, 38, 39 to 100% by weight of hydrogel forming polymer accordingto the invention, the even more preferred range 40, 41, 42, 43, 44, 45,46, 47, 48, 49 to 100% by weight of hydrogel forming polymer accordingto the invention, the much more preferred range 50, 51, 52, 53, 54, 55,56, 57, 58, 59 to 100% by weight of hydrogel forming polymer accordingto the invention, the particularly preferred range 60, 61, 62, 63, 64,65, 66, 67, 68, 69 to 100% by weight of hydrogel forming polymeraccording to the invention, the especially preferred range 70, 71, 71,72, 73, 74, 75, 76, 77, 78, 79 to 100% by weight of hydrogel formingpolymer according to the invention and the most preferred range 90, 91,92, 93, 94, 95, 96, 97, 98, 99 or 100% by weight of hydrogel formingpolymer according to the invention.

[0275] Hygiene articles for the purposes of the present inventioninclude not only incontinence pads and incontinence briefs for adultsbut also diapers for infants.

[0276] The liquid pervious topsheet (A) is the layer which is in directcontact with the skin of the wearer. Its material comprises customarysynthetic or manufactured fibers or films of polyesters, polyolefins,rayon or natural fibers such as cotton. In the case of non-wovenmaterials the fibers are generally joined together by binders such aspolyacrylates. Preferred materials are polyesters, rayon or blendsthereof, polyethylene and polypropylene. Examples of liquid perviouslayers are described in WO 99/57355 A1, EP 102 388 3 A2.

[0277] The liquid impervious layer (B) is generally a sheet ofpolyethylene or polypropylene.

[0278] The core (C) includes not only the hydrogel forming polymer ofthe invention but also hydrophilic fiber material. By hydrophilic ismeant that aqueous fluids spread quickly over the fiber. The fibermaterial is usually a cellulose, modified cellulose, rayon, polyestersuch as polyethylene terephthlate. Particular preference is given tocellulose fibers such as pulp. The fibers generally have a diameter of1-200 μm, and preferably 10-100 μm, and also have a minimum length of 1mm.

[0279] Diaper construction and shape is common knowledge and describedfor example in WO 95/26 209 page 66 line 34 to page 69 line 11, DE 19604 601 A1, EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygienearticles are generally also described in WO 00/65084, especially atpages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502,especially pages 3-9, DE 19737434, WO 98/8439. Hygiene articles forfemcare are described in the following references. The inventivehydrogel forming polymers capable of absorbing aqueous fluids can beused therein. Femcare references: WO 95/24173: Absorption Article forControlling Odour, WO 91/11977: Body Fluid Odour Control, EP 389023:Absorbent Sanitary Articles, WO 94/25077: Odour Control Material, WO97/01317: Absorbent Hygienic Article, WO 99/18905, EP 834297, U.S. Pat.No. 5,762,644, U.S. Pat. No. 5,895,381, WO 98/57609, WO 2000/065083, WO2000/069485, WO 2000/069484, WO 2000/069481, U.S. Pat. No. 6,123,693, EP1104666, WO 2001/024755, WO 2001/000115, EP 105373, WO 2001/041692, EP1074233. Tampons are described in the following references: WO98/48753., WO 98/41179, WO 97/09022, WO 98/46182, WO 98/46181, WO2001/043679, WO 2001/043680, WO 2000/061052, EP 1108408, WO 2001/033962,DE 200020662, WO 2001/001910, WO 2001/001908, WO 2001/001909, WO2001/001906, WO 2001/001905, WO 2001/24729. Incontinence articles aredescribed in the following references: Disposable. Absorbent Article forIncontinent Individuals: EP 311344 description pages 3-9; DisposableAbsorbent Article: EP 850623; Absorbent Article: WO 95/26207; AbsorbentArticle: EP 894502; Dry Laid Fibrous Structure: EP 850 616; WO 98/22063;WO 97/49365; EP 903134; EP 887060; EP 887059; EP 887058; EP 887057; EP887056; EP 931530; WO 99/25284; WO 98/48753. Femcare and incontinencearticles are described in the following references: Catamenial Device:WO 93/22998 description pages 26-33; Absorbent Members for Body Fluids:WO 95/26209 description pages 36-69; Disposable Absorbent Article: WO98/20916 description pages 13-24; Improved Composite AbsorbentStructures: EP 306262 description pages 3-14; Body Waste AbsorbentArticle: WO 99/45973. These references and the references therein arehereby expressly incorporated herein.

[0280] The acidic hydrogel forming polymers of the invention are veryuseful as absorbents for water and aqueous fluids, so that they may beused with advantage as a water retainer in market gardening, as a filteraid and particularly as an absorbent component in hygiene articles suchas diapers, tampons or sanitary napkins.

[0281] Incorporation and Fixation of the Highly Swellable HydrogelsAccording to the Present Invention.

[0282] In addition to the above-described highly swellable hydrogels,the absorbent composition of the present invention includesconstructions which include highly swellable hydrogels or to which theyare fixed. Any construction is suitable that is capable of accommodatinghighly swellable hydrogels and of being integrated into the absorptionlayer. A multiplicity of such compositions is already known anddescribed in detail in the literature. A construction for installing thehighly swellable hydrogels can be for example a fiber matrix consistingof a cellulose fiber mixture (air-laid web, wet laid web) or syntheticpolymer fibers (meltblown web, spunbonded web) or else of a fiber blendof cellulose fibers and synthetic fibers. Possible fiber materials aredetailed in the chapter which follows. The air-laid web process isdescribed for example in WO 98/28 478. Furthermore, open-celled foams orthe like may be used to install highly swellable hydrogels.

[0283] Alternatively, such a construction can be the result of fusingtwo individual layers to form one or better a multiplicity of chamberswhich contain the highly swellable hydrogels. Such a chamber system isdescribed in detail in EP 0 615 736 A1 page 7 lines 26 et seq.

[0284] In this case, at least one of the two layers should be waterpervious. The second layer may either be water pervious or waterimpervious. The layer material used may be tissues or other fabric,closed or open-celled foams, perforated films, elastomers or fabricscomposed of fiber material. When the absorbent composition consists of aconstruction of layers, the layer material should have a pore structurewhose pore dimensions are small enough to retain the highly swellablehydrogel particles. The above examples of the construction of theabsorbent composition also include laminates composed of at least twolayers between which the highly swellable hydrogels are installed andfixed.

[0285] Generally it is possible to fix hydrogel particles within theabsorbent core to improve dry and wet integrity. Dry and wet integritydescribes the ability to install highly swellable hydrogels into theabsorbent composition in such a way that they withstand external forcesnot only in the wet but also in the dry state and highly swellablepolymer does not dislocate or spill out. The forces referred to areespecially mechanical stresses as occur in the course of moving aboutwhile wearing the hygiene article or else the weight pressure on thehygiene article in the case of incontinence especially. As to fixation,one skilled in the art knows a multiplicity of possibilities. Examplessuch as fixation by heat treatment, addition of adhesives,thermoplastics, binder materials are noted in WO 95/26 209 page 37 line36 to page 41 line 14. The cited passage is thus part of this invention.Methods for enhancing wet strength are also to be found in WO 2000/36216A1.

[0286] Furthermore, the absorbent composition may comprise a basematerial, for example a polymer film on which the highly swellablehydrogel particles are fixed. The fixing may be effected not only on oneside but also on both sides. The base material can be water pervious orwater impervious.

[0287] The above constructions of the absorbent composition incorporatethe highly swellable hydrogels at a weight fraction of from 10 to 100%by weight, preferably 20-100% by weight, more preferably 30-100% byweight, even more preferably 40-100% by weight, much more preferably50-100% by weight, particularly preferably 60-100% by weight, especiallypreferably 70-100% by weight, extremely preferably 80-100% by weight andmost preferably 90-100% by weight, based on the total weight of theconstruction and of the highly swellable hydrogels.

[0288] Fiber Materials of the Absorbent Composition

[0289] The structure of the present composition according to theinvention may be based on various fiber materials, which are used as afiber network or matrices. The present invention includes not onlyfibers of natural origin (modified or unmodified) but also syntheticfibers.

[0290] A detailed overview of examples of fibers which can be used inthe present invention is given in WO 95/26 209 page 28 line 9 to page 36line 8. The cited passage is thus part of this invention.

[0291] Examples of cellulose fibers include cellulose fibers which arecustomarily used in absorption products, such as fluff pulp andcellulose of the cotton type. The materials (soft- or hardwoods),production processes such as chemical pulp, semichemical pulp,chemothermomechanical pulp (CTMP) and bleaching processes are notparticularly restricted. For instance, natural cellulose fibers such ascotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate areused.

[0292] Suitable synthetic fibers are produced from polyvinyl chloride,polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride,polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinylacetate, soluble or insoluble polyvinyl alcohol. Examples of syntheticfibers include thermoplastic polyolefin fibers, such as polyethylenefibers (PULPEX®), polypropylene fibers and polyethylene-polypropylenebicomponent fibers, polyester fibers, such as polyethylene terephthalatefibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride,polyacrylics, polyamides, copolyamides, polystyrene and copolymers ofthe aforementioned polymers and also bicomponent fibers composed ofpolyethylene terephthalate-polyethylene-isophthalate copolymer,polyethyl vinyl acetate/polypropylene, polyethylene/polyester,polypropylene/polyester, copolyester/polyester, polyamide fibers(nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrilefibers. Preference is given to polyolefin fibers, polyester fibers andtheir bicomponent fibers. Preference is further given to thermallyadhesive bicomponent fibers composed of polyolefin of the core-sheathtype and side-by-side type on account of their excellent dimensionalstability following fluid absorption.

[0293] The synthetic fibers mentioned are preferably used in combinationwith thermoplastic fibers. In the course of the heat treatment, thelatter migrate to some extent into the matrix of the fiber materialpresent and so constitute bond sites and renewed stiffening elements oncooling. Additionally the addition of thermoplastic fibers means thatthere is an increase in the present pore dimensions after the heattreatment has taken place. This makes it possible, by continuousaddition of thermoplastic fibers during the formation of the absorbentcore, to continuously increase the fraction of thermoplastic fibers inthe direction of the topsheet, which results in a similarly continuousincrease in the pore sizes. Thermoplastic fibers can be formed from amultiplicity of thermoplastic polymers which have a melting point ofless than 190° C., preferably in the range from 75° C. to 175° C. Thesetemperatures are too low for damage to the cellulose fibers to belikely.

[0294] Lengths and diameters of the above-described synthetic fibers arenot particularly restricted, and generally any fiber from 1 to 200 mm inlength and from 0.1 to 100 denier (gram per 9 000 meters) in diametermay preferably be used. Preferred thermoplastic fibers are from 3 to 50mm in length, particularly preferred thermoplastic fibers are from 6 to12 mm in length. The preferred diameter for the thermoplastic fiber isin the range from 1.4 to 10 decitex, and the range from 1.7 to 3.3decitex (gram per 10 000 meters) is particularly preferred. The form ofthe fiber may vary; examples include woven types, narrow cylindricaltypes, cut/chopped yarn types, staple fiber types and continuousfilament fiber types.

[0295] The fibers in the absorbent composition of the present inventioncan be hydrophilic and/or hydrophobic. According to the definition ofRobert F. Gould in the 1964 American Chemical Society publication“Contact angle, wettability and adhesion”, a fiber is referred to ashydrophilic when the contact angle between the liquid and the fiber (orthe fiber surface) is less than 90° or when the liquid tends to spreadspontaneously on the same surface. The two processes are generallycoexistent. Conversely, a fiber is termed hydrophobic when a contactangle of greater than 90° is formed and no spreading is observed.

[0296] Preference is given to using hydrophilic fiber material.Particular preference is given to using fiber material which is weaklyhydrophilic on the body side and most hydrophilic in the regionsurrounding the highly swellable hydrogels. In the manufacturingprocess, layers having different hydrophilicities are used to create agradient which channels impinging fluid to the hydrogel, where it isultimately absorbed.

[0297] Suitable hydrophilic fibers for use in the absorbent compositionof the present invention include for example cellulose fibers, modifiedcellulose fibers, rayon, polyester fibers, for example polyethyleneterephthalate (DACRON®), and hydrophilic nylon (HYDROFIL®). Suitablehydrophilic fibers may also be obtained by hydrophilicizing hydrophobicfibers, for example the treatment of thermoplastic fibers obtained frompolyolefins (e.g. polyethylene or polypropylene, polyamides,polystyrenes, polyurethanes, etc.) with surfactants or silica. However,for cost reasons and ease of availability, cellulosic fibers arepreferred.

[0298] The highly swellable hydrogel particles are embedded into thefiber material described. This can be done in various ways, for exampleby using the hydrogel material and the fibers together to create anabsorbent layer in the form of a matrix, or by incorporating highlyswellable hydrogels into fiber mixture layers, where they-are ultimatelyfixed, whether by means of adhesive or lamination of the layers.

[0299] The fluid-acquiring and -distributing fiber matrix may comprisesynthetic fiber or cellulosic fiber or a mixture of synthetic fiber andcellulosic fiber, in which case the mixing ratio may vary from (100 to0) synthetic fiber: (0 to 100) cellulosic fiber. The cellulosic fibersused may additionally have been chemically stiffened to increase thedimensional stability of the hygiene article.

[0300] The chemical stiffening of cellulosic fibers may be provided indifferent ways. A first way of providing fiber stiffening is by addingsuitable coatings to the fiber material. Such additives include forexample polyamide-epichlorohydrin coatings (Kymene® 557 H, Hercoles,Inc. Wilmington, Del.), polyacrylamide coatings (described in U.S. Pat.No. 3,556,932 or as the Parez® 631 NC commercial product from AmericanCyanamid Co., Stamford, Conn.), melamine-formaldehyde coatings andpolyethyleneimine coatings.

[0301] Cellulosic fibers may also be chemically stiffened by chemicalreaction. For instance, suitable crosslinker substances may be added toeffect crosslinking taking place within the fiber. Suitable crosslinkersubstances are typical substances used for crosslinking monomersincluding but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydeshaving acid functionality and in particular C₂-C₉-polycarboxylic acids.Specific substances from this series are for example glutaraldehyde,glyoxal, glyoxylic acid, formaldehyde and citric acid. These substancesreact with at least 2 hydroxyl groups within any one cellulose chain orbetween two adjacent cellulose chains within any one cellulose fiber.The crosslinking causes a stiffening of the fibers, to which greaterdimensional stability is imparted as a result of this treatment. Inaddition to their hydrophilic character, these fibers exhibit uniformcombinations of stiffening and elasticity. This physical property makesit possible to retain the capillary structure even under simultaneouscontact with fluid and compressive forces and to prevent prematurecollapse.

[0302] Chemically crosslinked cellulose fibers are known and describedin WO 91/11162, U.S. Pat. No. 3,224,926, U.S. Pat. No. 3,440,135, U.S.Pat. No. 3,932,209, U.S. Pat. No. 4,035,147, U.S. Pat. No. 4,822,453,U.S. Pat. No. 4,888,093, U.S. Pat. No. 4,898,642 and U.S. Pat. No.5,137,537. The chemical crosslinking imparts stiffening to the fibermaterial, which is ultimately reflected in improved dimensionalstability for the hygiene article as a whole. The individual layers arejoined together by methods known to one skilled in the art, for exampleintermelting by heat treatment, addition of hot-melt adhesives, latexbinders, etc.

[0303] Methods of Making the Absorbent Composition

[0304] The absorbent composition is composed of constructions whichcontain acidic highly swellable hydrogels and the acidic highlyswellable hydrogels which are present in said constructions or fixedthereto.

[0305] Examples of processes to obtain an absorbent compositioncomprising for example a base material to which highly swellablehydrogels are fixed on one or both sides are known and included by theinvention but not limited thereto.

[0306] Examples of processes to obtain an absorbent compositioncomprising for example highly swellable hydrogels (c) embedded in afiber material blend of synthetic fibers (a) and cellulose fibers (b),the blend ratio varying from (106 to 0) synthetic fiber : (0 to 100)cellulose fiber, include (1) a process where (a), (b) and (c) are mixedtogether at one and the same time, (2) a process where a mixture of (a)and (b) is mixed into (c), (3) a process where a mixture of (b) and (c)is mixed with (a), (4) a process where a mixture of (a) and (c) is mixedinto (b), (5) a process where (b) and (c) are mixed and (a) iscontinuously metered in, (6) a process where (a) and (c) are mixed and(b) is continuously metered in, and (7) a process where (b) and (c) aremixed separately into (a). Of these examples, processes (1) and (5) arepreferred. The apparatus used in this process is not particularlyrestricted and any customary apparatus known to one skilled in the artcan be used.

[0307] The absorbent composition obtained in this way can optionally besubjected to a heat treatment, so that an absorption layer havingexcellent dimensional stability in the moist state is obtained.

[0308] The heat treatment process is not particularly restricted.Examples include heat treatment by feeding hot air or infraredirradiation. The temperature of the heat treatment is in the range from60° C. to 230° C., preferably from 100° C. to 200° C., particularlypreferably from 100° C. to 180° C.

[0309] The duration of the heat treatment depends on the type ofsynthetic fiber, its amount and the hygiene article production rate.Generally the duration of the heat treatment is in the range from 0.5second to 3 minutes, preferably from 1 second to 1 minute.

[0310] The absorbent composition is generally provided for example witha liquid-pervious topsheet and a liquid-impervious backsheet.Furthermore, leg cuffs and adhesive tabs are attached to finalize thehygiene article. The materials and types of pervious topsheet andimpervious backsheet and of the leg cuffs and adhesive tabs are known toone skilled in the art and are not particularly restricted. Examplesthereof may be found in WO 95/26 209.

[0311] Experimental Part

[0312] Test Methods

[0313] a) Centrifuge Retention Capacity (CRC)

[0314] This method measures the free swellability of the hydrogel in ateabag. 0.2000±0.0050 g of dried hydrogel (particle size fraction106-850 μm) are weighed into a teabag 60×85 mm in size which issubsequently sealed. The teabag is placed for 30 minutes in an excess of0.9% by weight sodium chloride solution (at least. 0.83 l of sodiumchloride solution/1 g of polymer powder). The teabag is then centrifugedfor 3. minutes at 250 g. The amount of liquid is determined by weighingback the centrifuged teabag.

[0315] b) Absorbency Under Load (AUL) (0.7 psi)

[0316] The measuring cell for determining AUL 0.7 psi is a Plexiglasscylinder 60 mm in internal diameter and 50 mm in height. Adhesivelyattached to its underside is a stainless steel sieve bottom having amesh size of 36 μm. The measuring cell further includes a plastic platehaving a diameter of 59 mm and a weight which can be placed in themeasuring cell together with the plastic plate. The plastic plate andthe weight together weigh 1,345 g. AUL 0.7 psi is determined bydetermining the weight of the empty Plexiglass cylinder and of theplastic plate and recording it as W₀. 0.900±0.005 g of hydrogel formingpolymer (particle size distribution 150-800 μm) is then weighed into thePlexiglass cylinder and distributed very uniformly over the stainlesssteel sieve bottom. The plastic plate is then carefully placed in thePlexiglass cylinder, the entire unit is weighed and the weight isrecorded as W_(a). The weight is then placed on the plastic plate in thePlexiglass cylinder. A ceramic filter plate 120 mm in diameter and 0 inporosity is then placed in the middle of a Petri dish 200 mm in diameterand 30 mm in height and sufficient 0.9% by weight sodium chloridesolution is introduced for the surface of the liquid to be level withthe filter plate surface without the surface of the filter plate beingwetted. A round filter paper 90 mm in diameter and <20 μm in pore size(S&S 589 Schwarzband from Schleicher & Schüll) is subsequently placed onthe ceramic plate. The Plexiglass cylinder containing hydrogel formingpolymer is then placed with plastic plate and weight on top of thefilter paper and left there for 60 minutes. At the end of this period,the complete unit is removed from the Petri dish and subsequently theweight is removed from the Plexiglass cylinder. The Plexiglass cylindercontaining swollen hydrogel is weighed together with the plastic plateand the weight recorded as W_(b).

[0317] AUL was calculated by the following equation:

AUL 0.7 psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]

[0318] c) Saline Flow Conductivity (SFC)

[0319] The test method for determining SFC is described in U.S. Pat. No.5,599,335.

[0320] d) pH Measurement of Hydrogel Forming Polymers

[0321] 100 ml of 0.9% by weight NaCl solution is magnetically stirred atmoderate speed in a 150 ml beaker without air being drawn into thesolution. This solution is admixed with 0.5±0.001 g of hydrogel formingpolymer and stirred for 10 minutes. After 10 minutes, the pH of thesolution is measured with a pH glass electrode, the value not being readoff until it is stable, but at the earliest after 1 minute.

[0322] e) Ammonia Determination for Odor Control

[0323] The ammonia nitrogen content is determined calorimetrically bythe Nessler method. Urea eliminates ammonia under the action of urease;a yellow color develops to a degree proportional to the ammoniaconcentration.

[0324] 5 g of the various superabsorbent samples were saturated with 600ml of 0.9% NaCl and 1.8% urea solution for 20 min. The solutions werefiltered and 25 ml of the solution were admixed with 10 μl of ureasesolution. After 2 minutes nitrogen from ammonia was determined by theNessler method.

EXAMPLES Example 1a

[0325] A Werner & Pfleiderer laboratory kneader having a workingcapacity of 2 l is evacuated to 980 mbar absolute by means of a vacuumpump and a previously separately prepared monomer solution which hasbeen cooled to about 25° C. and inertized by passing nitrogen into it issucked into the kneader. The monomer solution has the followingcomposition: 825.5 g of completely ion-free water, 431 g of acrylicacid, 120.68 g of NaOH 50%, 0.86 g of polyethylene glycol 400 diacrylate(SARTOMER® 344 from Cray Valley). To improve the inertization, thekneader is evacuated and subsequently refilled with nitrogen. Thisoperation is repeated three times. A solution of 1.2 g of sodiumpersulfate (dissolved in 6.8 g of completely ion-free water) is thensucked in, followed after a further 30 seconds by a further solutionconsisting of 0.024 g of ascorbic acid dissolved in 4.8 g of completelyion-free water. After a nitrogen purge a preheated jacket heatingcircuit on bypass at 75° C. is switched over to the kneader jacket andthe stirrer speed increased to 96 rpm. Following the onset ofpolymerization and the attainment of T_(max), the jacket heating circuitis switched back to bypass, and the batch is supplementarily polymerizedfor 15 minutes without heating/cooling, subsequently cooled anddischarged. The resultant gel particles are dried at 160° C. on wiremesh bottomed trays in a through air drying cabinet and then ground andsieved.

Example 1b

[0326] 1 200 g of the thus obtained product of particle sizedistribution 105-850 μm were sprayed with a homogeneous solutionconsisting of 20 g of water, 0.18 g of ethylene glycol diglycidyl etherand 0.6 g of sorbitan monococoate in a powder mixing assembly (Loedigemixer) and transferred into a preheated and 2nd Loedige mixer. The heattreatment was carried out under constant conditions at a jackettemperature of 150° C. and a speed of 60 rpm for a period of 120minutes. The mixer was emptied, and the product was cooled down to roomtemperature and sieved off at 105/850 μm to remove any agglomerates orfines which may have formed. The performance data are shown in Table 1.

Example 1c

[0327] Example 1b was repeated except that the heat treatment wascarried out for 70 minutes only and the postcrosslinking solution for 1200 g of powder from Example 1a had the following composition: 17.58 gof water, 9.96 g of 1,2-propanediol, 1,2 g of ethylene glycol diglycidylether and 3.36 g of aqueous 26.8% aluminum sulfate solution. Theperformance data are shown in Table 1.

Example 1d

[0328] Example 1b was repeated except that the heat treatment wascarried out for 15 minutes only and the postcrosslinking solution for 1200 g of powder from Example 1a had the following composition: 20.00 gof water and 0.90 g of ethylene glycol diglycidyl ether. The performancedata are shown in Table 1.

Example 2

[0329] A 10 l capacity polyethylene vessel thoroughly insulated withfoamed plastic material is charged with 3 928 g of completely ion-freewater, 630 g of sodium bicarbonate are suspended in the water, and 2 000g of acrylic acid are added with stirring in such a way that thereaction solution does not foam over as a result of the onset of CO₂evolution. This is followed by the addition, in succession, of anemulsion of 1.3 g of sorbitan monococoate in 100 g of completelyion-free water and of 6.00 g of allyl methacrylate, and the solution isfurther inertized by passing nitrogen into it. This is followed by theaddition of the initiator system, consisting of 1.66 g of2,2′-azobisamidinopropane dihydrochloride (dissolved in 20 g ofcompletely ion-free water), 3.33 g of potassium peroxodisulfate(dissolved in 150 g of completely ion-free water) and also 0.3 g ofascorbic acid (dissolved in 25 g of completely ion-free water) insuccession with stirring. The reaction solution is then left to standwithout stirring. The polymerization which ensues, and in the course ofwhich the temperature rises to about 90° C., produces a solid gel. Thissolid gel is mechanically comminuted using a meat grinder, dried at 135°C. in a through air cabinet on VA stainless steel wire mesh and thenground and sieved.

Example 2a

[0330] Product of Example 2 is postcrosslinked similarly to Example 1busing a postcrosslinking solution which had the following compositionfor 1 000 g of polymer: 19.26 g of water, 19.50 g of 1,2-propanediol,0.25 g of 2-oxazolidinone, 0.49 g of aluminum sulfate octadecahydrate.The performance data after a residence time of 30 minutes are shown inTable 1.

Example 2b

[0331] Product of Example 2 is postcrosslinked similarly to Example 2a.The performance data after a residence time of 60 minutes are shown inTable 1.

Example 2c

[0332] The product of Example 2 is postcrosslinked similarly to Example2a. The performance data after a residence time of 70 minutes are shownin Table 1.

Example 3

[0333] A 10 l capacity polyethylene vessel thoroughly insulated withfoamed plastic material is charged with 4 046 g of completely ion-freewater, 408 g of lithium hydroxide 1-hydrate are dissolved therein, and 2000 g of acrylic acid are slowly added with stirring. This is followedby the addition, in succession, of an emulsion of 1.3 g of sorbitanmonococoate in 100 g of completely ion-free water and of 8.1 g of allylmethacrylate, and the solution is further inertized by passing nitrogeninto it. This is followed by the addition of the initiator system,consisting of 1.66 g of 2,2′-azobisamidinopropane dihydrochloride(dissolved in 20 g of completely ion-free water), 3.33 g of potassiumperoxodisulfate (dissolved in 150 g of completely ion-free water) andalso 0.3 g of ascorbic acid (dissolved in 25 g of completely ion-freewater) in succession with stirring. The reaction solution is thenallowed to stand without stirring. The polymerization which ensues, andin the course of which the temperature rises to about 90° C., produces asolid gel. This solid gel is mechanically comminuted using a meatgrinder. 150 g of the gel thus comminuted are placed in a metal cylinderhaving an internal diameter of 10 cm and a VA stainless steel wire meshbottom, dried therein under a 140° C. 3.11 m/sec hot air stream attemperatures of . . . in the course of 25 minutes, subsequently groundand sieved.

Example 3a

[0334] Product of Example 3 is admixed on a 20 g scale in a Waringblender (modified attachment for kitchen blender) with a surfacepostcrosslinking solution (sprayed from a 2 ml hypodermic) consisting of3.35% of water/1.65% of 1,2-propanediol/0.03% of ethylene glycoldiglycidyl ether and 0.075% of aluminum sulfate (each based on polymer)and heat treated at 100° C. in a through air cabinet for 2 hours. Theperformance data are shown in Table 1.

Example 4

[0335] A 10 l capacity polyethylene vessel thoroughly insulated withfoamed plastic material is charged with 3 944 g of completely ion-freewater, 625 g of sodium bicarbonate are suspended in the water, and 1 400g of acrylic acid are added with stirring in such a way that thereaction solution does not foam over as a result of the onset of CO₂evolution. This is followed by the addition, in succession, of 600 g of2-acrylamido-2-methylpropanesulfonic acid and also of an emulsion of 1.3g of sorbitan monococoate in 100 g of completely ion-free water and of6.5 g of allyl methacrylate, and the solution is further inertized bypassing nitrogen into it. This is followed by the addition of theinitiator system, consisting of 1.66 g of 2,2′-azobisamidinopropanedihydrochloride (dissolved in 20 g of completely ion-free water), 3.33 gof potassium peroxodisulfate (dissolved in 150 g of completely ion-freewater) and also 0.3 g of ascorbic acid (dissolved in 25 g of completelyion-free water) in succession with stirring. The reaction solution isthen left to stand without stirring. The polymerization which ensues,and in the course of which the temperature rises to about 90° C.,produces a solid gel. This solid gel is mechanically comminuted using ameat grinder. 150 g of the gel thus comminuted are placed in a metalcylinder having an internal diameter of 10 cm and a VA stainless steelwire mesh bottom, dried therein under a 140° C. 3.1 m/sec hot air streamat temperatures of . . . in the course of 25 minutes, subsequentlyground and sieved.

Example 4a

[0336] Product of Example 4 is similarly to Example 3a on a 20 g scalein a Waring blender with a surface postcrosslinking solution consistingof 1.95% of water/1.95% of 1,2-propanediol/0.075% of ethylene glycoldiglycidyl ether and 0.075% of aluminum sulfate (each based on polymer)and heat treated at 140° C. in a through air drying cabinet for 2 hours.The performance data are shown in Table 1.

Example 5

[0337] A superabsorbent characterized by a pH of 5-5.5, preparedsimilarly to Example 7 of EP 0 316 792 B1, was admixed on a 20 g scalein a Waring blender (modified attachment for kitchen blender) with asurface postcrosslinking solution (sprayed from a 2 ml hypodermic)consisting of 2.3% of water/1.2% of 1,2-propanediol/0.2% of ethyleneglycol diglycidyl ether (each based on polymer) and heat treated at 140°C. in a through air drying cabinet for 1 hour. The performance data areshown in Table 1.

Example 6

[0338] 369.97 of acrylic acid (AA) is admixed with 2.94 g of ETMPTA(ethoxylated trimethylolpropane triacrlyate) (corresponds to 0.06 mol %based on AA). This solution is added with stirring to 184.83 g of a 50%by weight aqueous solution of NaOH (45 mol % based on AA) in 868.17 g ofdeionized water with cooling. The temperature is measured. At the start,AA is rapidly admixed with ETMPTA until the temperature has reachedabout 50° C. Thereafter AA has ETMPTA (up to about 75%) added to itdropwise so that the temperature remains at about 50° C.±5° C. Theremaining AA can be rapidly admixed with ETMPA. The monomer solution iscooled to 10° C. in an ice bath and admixed with 0.09 g of Darocur 1173:Irgacure 651 (2:1 ratio) photoinitiators, which are dissolved withstirring. The monomer solution is cooled again to 10° C. in an ice bathand admixed with 6.832 g of 10% sodium persulfate solution (0.07 mol %based on AA). The monomer solution is added in a 1.5 L glass vessel withtemperature control. The vessel is placed under a UV emission lamp (20mWcm⁻² measured at vessel base). An exothermic polymerization ismeasured for 7.5 minutes, after which a transparent gel can be removedfor further processing.

[0339] 1 kg of the SAP gel obtained were comminuted for 30 s in a 2 Ltwin sigma blade mixer to enlarge the surface area and mixed with 100 gof fine SAP powder (particle size <106 um) for a further 90 s. Asolution of 26 g of 1% sodium metabisulfite (0.026 mol % based on AA)was added and mixed in for a further 90 s. This was followed by theaddition of 20 g of 1% Span 20 solution (sorbitan monolaurate) (0.02% byweight based on gel) and mixed in for 15 s.

[0340] The comminuted gel is extruded in an extruder having a 4 mm dieplate to enlarge the surface area and subsequently dried on a metalplate having 3 mm holes for air circulation at a gel thickness of 2-4 mmat 160° C. for 45 min.

[0341] The dried material is comminuted in a hammer mill and theparticle size is adjusted by sieving to 180-710 μm.

[0342] The subsequent surface crosslinking is carried out with 4% byeight of a solution containing 1.5% of ethylene glycol diglycidyl ether(EGDGE—Nagase Chemicals Japan as Denacol Ex-810), 36.94% of propyleneglycol and 61.56% of water by drying and crosslinking at 145° C. for 60min.

Example 7

[0343] Similar to Example 6 except that 205.37 g of 50% NaOH solution in847.63 g of water are introduced as initial charge.

[0344] The performance data are shown in Table 1.

Example 8a

[0345] A Werner & Pfleiderer laboratory kneader having a workingcapacity of 2 1 is evacuated to 50 mbar by means of vacuum and apreviously separately prepared monomer solution which has been cooled toabout 25° C. and inertized by passing nitrogen into it is sucked intothe kneader. The monomer solution has the following composition: 3095.7g of completely ion-free water, 1821.68 g of acrylic acid, 1012.05 g ofNaOH 50%, 8.20 g of polyethylene glycol 400 diacrylate (Sartomer® 344from CRAY VALLEY) and also 1.46 g of sorbitan monococoate. To improvethe inertization, the kneader is evacuated and subsequently refilledwith nitrogen. This operation is repeated 3 times. Then 35.22 g of anaqueous 15% sodium persulfate solution, 0.18 g of a 1% aqueous hydrogenperoxide solution and, 30 seconds later, 25.50 g of a 0.5% aqueousascorbic acid solution is sucked in. After a nitrogen purge a preheatedjacket heating circuit on bypass at 75° C. is switched over to thekneader jacket and the stirrer speed raised to 96 rpm. Following theonset of polymerization and the attainment of T_(max), the jacketheating circuit is switched back to bypass, and the batch issupplementarily polymerized for 15 minutes without heating/cooling,subsequently cooled and discharged. The resultant gel particles aredried at 160° C. on wire mesh bottomed trays in a through air dryingcabinet and then ground and sieved.

Example 8b

[0346] 1200 g of a thus obtained product of particle size distribution105-850 μm were sprayed with a homogeneous solution consisting of 39.5 gof water, 19.7 g of 1,2-propanediol and 0.72 g of ethylene glycoldiglycidyl ether in a powder mixing assembly (Loedige mixer) andtransferred into a second, preheated Loedige mixer. The heat treatmentwas carried out under constant conditions at a jacket temperature of150° C. and a speed of 60 rpm for a period of 120 minutes. The mixer wasemptied, and the product was cooled down to room temperature and sievedoff at 105-850 μm to remove any agglomerates or fines which may haveformed. The performance data are shown in Table 1.

Example 9

[0347] Superabsorbents as described in Example 1 of WO 00/22018 page 14.TABLE 1 CRC AUL Example pH SFC × 10⁻⁷ cm³s/g g/g 0.7 psi g/g PH_(AI) 1a4.44 1 23.2 5.8 74.2 1b 4.45 14.5 21.4 14.5 91.5 1c 4.47 13.8 20.7 18.198.2 1d 4.45 12 21.9 14.7 93.3 2  4.39 1 27.6 5.0 85.1 2a 4.39 16 25.113.0 99.4 2b 4.39 24 23.3 15.8 102 2c 4.39 33 22.4 18.1 105.7 3  4.68 132.4 4.9 86.5 3a 4.69 9 29.0 13.9 99 4  4.37 2 30.0 4.8 91.5 4a 4.39 3122.4 15.7 99.4 Example 7 of 5.4 ≦1 42.0 6.0 76.8 EP 316 792 notinventive 5  5.4 17 32.9 23.0 89.4 6  5.1 18 29.7 26.4 106.6 7  5.3 1630.7 26.8 97.8 8  5.2 28.2 26.1 97.7

[0348] Table 2 summarizes the experimental results of the ammonianitrogen determination of the products having different pH values fromthe examples. Table 2 demonstrates the odor binding effect of acidicsuperabsorbent products. TABLE 2 N₂ from NH₃ (Nessler) Nessler ExamplepH mg/1 value  1c 4.47 1.8 20% 6 5.1 2.1 23% 7 5.3 3.9 43% 5 5.4 5.6 62%9 6.1 9.0 100% 

1. Hydrogel forming polymers capable of absorbing aqueous fluids andhaving a pH absorbency index PH_(AI), which is calculated as follows: pH_(AI) =ΔpH·(AUL _(0.7psi) +CRC) where ΔpH=7−pH of product AUL_(0.7psi)is the absorbency under pressure at 0.7 psi CRC is the centrifugeretention capacity of at least 80, preferably at least 90 and especiallyat least 100, and having a pH of not more than 5.7:
 2. Hydrogel formingpolymers as claimed in claim 1, having a pH of from 4 to 5.5 andespecially from 4.4 to 4.6 or from 5.1 to 5.3.
 3. Hydrogel formingpolymers as claimed in claim 1 or 2, having a Nessler value of less than60%, preferably less than 40% and especially less than 30%.
 4. Hydrogelforming polymers as claimed in any of claims 1 to 3, having a CRCgreater than 15 g/g and preferably greater than 26 g/g.
 5. Hydrogelforming polymers as claimed in any of claims 1 to 4, having an AUL 0.7psi greater than 13 g/g and preferably greater than 15 g/g.
 6. Hydrogelforming polymers as claimed in any of claims 1 to 5, obtainable from abase polymer having a pH of not more than 5.7, preferably from 4 to 5.5and especially from 4.4 to 4.6 or from 5.1 to 5.3 by surfacepostcrosslinking.
 7. Hydrogel forming polymers as claimed in any ofclaims 1 to 6, being an acrylic acid (co)polymer partially neutralizedto an alkali metal, alkaline earth metal, ammonium or amine salt. 8.Hydrogel forming polymers as claimed in any of claims 1 to 7, havingantimicrobial properties.
 9. The use of hydrogel forming polymers asclaimed in any of claims 1 to 8 for preparing absorbent articles capableof permanently or temporarily binding aqueous fluids.
 10. A process forpreparing hydrogel forming polymers as claimed in any of claims 1 to 8,which comprises surface postcrosslinking a base polymer having a pH ofnot more than 5.7, preferably from 4 to 5.5 and especially from 4.4 to4.6 or from 5.1 to 5.3.
 11. Gel prepared by polymerization of partiallyneutralized acrylic acid with a crosslinking agent, wherein (i) thepartial neutralization can be effected using NaOH 50%: 20-30 mol % basedon acrylic acid, preferably 22-29 mol %, more preferably 23 to 28 mol %,particularly preferably 24 to 27 mol %, especially about 25.2 mol %based on acrylic acid, a corresponding neutralization can also beachieved with other neutralizing agents, the crosslinking can beeffected using polyethylene glycol 400 diacrylate: 0.005-1.0% by weightbased on acrylic acid, preferably 0.1-0.4% by weight, particularlypreferably 0.15-0.25% by weight, especially about 0.2% by weight basedon acrylic acid; or some other crosslinker which produces the samedegree of crosslinking, or (ii)the partial neutralization can beeffected using NaHCO₃: 23-28% by weight based on acrylic acid,preferably 27 to 35% by weight, particularly preferably 30 to 33% byweight, especially about 31.5% by weight based on acrylic acid, acorresponding neutralization can also be achieved with otherneutralizing agents, the crosslinking can be effected using allylmethacrylate: 0.005-1.0% by weight based on acrylic acid, preferably0.1-0.5% by weight, particularly preferably 0.2-0.4% by weight,especially about 0.3% by weight based on acrylic acid; or some othercrosslinker which produces the same degree of crosslinking, or (iii) thepartial neutralization can be effected using LiOH×H₂O: 15-30% by weightbased on acrylic acid, preferably 17 to 25% by weight, particularlypreferably 19 to 23% by weight, especially about 20.4% by weight basedon acrylic acid, a corresponding neutralization can also be achievedwith other neutralizing agents, the crosslinking can be effected usingallyl methacrylate: 0.005-1.0% by weight based on acrylic acid,preferably 0.1-0.6% by weight, particularly preferably 0.3-0.5% byweight, especially about 0.4% by weight based on acrylic acid; or someother crosslinker which produces the same degree of crosslinking, or(iv)the partial neutralization can be effected using NaHCO₃: 35-55% byweight based on acrylic acid, preferably 40 to 50% by weight,particularly preferably 43 to 46% by weight, especially about 44.5% byweight based on acrylic acid, a corresponding neutralization can also beachieved using other neutralizing agents when2-acrylamido-2-methylpropanesulfonic acid is present at 30-55% by weightbased on acrylic acid, preferably 35 to 50% by weight, particularlypreferably 41 to 45% by weight, especially about 43% by weight based onacrylic acid additionally to acrylic acid, the crosslinking canbe-effected using allyl methacrylate: 0.005-1.0% by weight based onacrylic acid, preferably 0.1-0.6% by weight, particularly preferably0.3-0.5% by weight, especially about 0.4% by weight based on acrylicacid, or some other crosslinker which produces the same degree ofcrosslinking.
 12. Base polymer obtainable by drying a gel mentioned inclaim
 11. 13. A process for screening superabsorbents, which comprisesdetermining the pH, CRC and AUL 0.7 psi of a plurality of superabsorbentsamples and determining the pH absorbency index therefrom.
 14. A processfor optimizing superabsorbents, which comprises iterating a process asclaimed in claim 13 and further varying the production parameters of thesamples having the highest pH absorbency indices.
 15. Hydrogel formingpolymers as claimed in any of claims 1 to 8, obtainable by a process asclaimed in claim
 14. 16. Hygiene articles comprising (A) a liquidpervious topsheet (B) a liquid impervious backsheet (C) a corepositioned between (A) and (B) and comprising 10-100% by weight of thehydrogel forming polymer 0-90% by weight of hydrophilic fiber materialas set forth in any of claims 1 to 8 or 15 (D) optionally a tissue layerpositioned directly above and below said core (C) and (E) optionally anacquisition layer positioned between (A) and (C).